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Amon, otherwise known as the Dark Voice, the Fallen One, the Dark One, the Dark God, the Eternal One, and Truth-bringer, was a malevolent xel'naga connected to the Void. He sought to break the "corrupt" cycle of the xel'naga and bring all life to an end, before remaking it in his image.
He has been described as "the universe's most ancient evil."
Amon was once a member of a species who possessed either purity of essence or form. In this form, Amon sought to become a great being, and he was given his chance. Per the Infinite Cycle of the xel'naga, he was imparted with the essence of the xel'naga elders and became one of them. Now xel'naga himself, Amon resented being torn from his previous life; he would later claim that the transformation had been forced on him. At the least, he felt lied to, and came to realize that greatness was not something he had truly desired and that he had become part of the Infinite Cycle himself.
At some point billions of years prior to the Great War, Amon became disillusioned with the Infinite Cycle of the xel'naga, viewing it as corrupt. In what would be the last universe that bore witness to the Cycle, while the majority of the xel'naga entered slumber in Ulnar after seeding the universe with life, Amon and a group of followers set out to secretly forge a single master race that would bring an end to the cycle.
While the xel'naga created life, their modus operandi was to let it develop on its own. Amon and his followers broke this rule by coming to the world of Aiur, which had given rise to a species called the protoss. Amon chose the protoss to be the race blessed with the Purity of Form. He and his followers subsequently uplifted the protoss, who came to revere the xel'naga as gods. 
At first, the protoss lived in harmony, bound by a communal psychic link. As time went on, however, the different tribes began to drift apart, each tribe seeking to define its own role in the universe. Frustrated by what they saw as a rise in independent egos, Amon and his followers worried that they had pushed the protoss' evolution too quickly. At the same time, the protoss started growing suspicious of Amon's true intentions, and the tribes began cultivating wild rumors about their creators' supposed treachery. Realizing that the protoss had grown beyond their control, Amon and the other xel'naga decided to leave Aiur. When the protoss learned of this, they recklessly attacked the xel'naga worldships, murdering hundreds of xel'naga in the process. Amon and his remaining followers fended off the protoss and fled the planet.
Despite the failure of the protoss, Amon still considered them key to his plans. In addition, he retained a number of protoss followers in the form of the Tal'darim, who separated themselves from their kindred. Amon promised the Tal'darim that through obedience, they would ascend into hybrid. In truth, this was a lie, a means to keep them under his thrall.
Having been driven from Aiur, Amon and his followers traveled to Zerus and began uplifting the zerg in order to create a race blessed with the purity of essence. Amon saw the zerg as a valuable tool, appreciating their ability to consume essence from other organisms. He thus bound them to the will of a hive mind, with only the primal zerg escaping his control. To avoid repeating the mistakes made with the protoss, Amon created the Overmind, a sentient entity that embodied the zerg hive mind. Although Amon gave the Overmind thought and reason, he did not give it free will. Instead, he forced upon it an overriding directive to destroy and assimilate the protoss, as part of his plans to forge the Hybrid.  He intended to enslave the zerg and use them and the hybrids to wipe out all life in the universe. Once the hybrids were dominant, he would dispose of the zerg and reshape all life in his own image.
War Among the Gods
"And the Swarm consumed the xel'naga..."
"Only a fraction remained within Ulnar. In the final confrontation, Amon was felled, but he was not defeated. He returned to the Void, twisting it with his hatred."
- Rohana and Artanis(src)
Before Amon could complete his work on Zerus, the other xel'naga at Ulnar awoke and learned of his plans. They waged a war against Amon and his followers that ultimately culminated in Amon unleashing the Zerg Swarm upon the xel'naga, destroying almost all of them. In the final battle, the last few surviving xel'naga at Ulnar defeated Amon. Although his body was destroyed, his consciousness escaped into the Void, where he began plotting his return. Amon still retained cognition and his influence on the Overmind and the Zerg Swarm remained. The zerg soon left Zerus to seek out and assimilate the protoss. At the same time, one of Amon's servants, who would come to be known as Samir Duran, began secretly working to artificially merge protoss and zerg DNA to form the desired hybrid race.  He also made plans to forge a new body for Amon.
The Countervailing Pawns
The Overmind was not blind to its plight. It foresaw the apocalyptic outcome if Amon, whom it called "the Fallen One", was victorious. Although unable to resist its directive to destroy the protoss, the Overmind sought to secretly undermine Amon's plan to enslave the Swarm. Under the pretense of creating a new weapon to combat the protoss, the Overmind had Sarah Kerrigan captured and transformed into the Queen of Blades.  The Overmind kept Kerrigan's true purpose hidden from everyone, including Kerrigan herself. In the future the Overmind glimpsed at, Amon had eventually recognized the threat that Kerrigan posed.
Kerrigan was reborn as the Queen of Blades after Amon's defeat at Ulnar. Because of this, while she still felt his lingering influence over her, and had her personality twisted, she was effectively an individual, as opposed to the Overmind. After the Brood War, Amon's will crept into her thoughts through the mutagen within her. She pulled the Swarm back to Char, and for four years, she quietly prepared the zerg as per Amon's will. However, while Kerrigan felt his influence, Amon never directly controlled her. Though convinced that the future could not be averted, Kerrigan resolved that she and the Swarm would fight to the death and perish in a final blaze of glory.
Although the zerg ultimately proved incapable of assimilating the protoss, Samir Duran, aided by terran mercenaries, successfully engineered Amon's protoss-zerg hybrids which he hid on numerous worlds. In the aftermath of the Brood War, Duran adopted the alias Emil Narud and allied himself with Valerian Mengsk. With Valerian's support, he established the Moebius Foundation. Using Moebius as a front for his operations, Narud continued breeding hybrids for Amon. Though Valerian knew of the existence of Narud's hybrids, he did not know the full extent of Narud's progress, nor his true motives.  At the same time, Narud collaborated with Emperor Arcturus Mengsk to establish the Dominion's secret Hybrid Breeding Program. Mengsk, unaware of Narud's true allegiance, intended to use the hybrids as weapons.
In the meantime, Amon continued to observe the Tal'darim. He watched a Rak'Shir in which 800 Tal'darim were killed through Alarak's manipulation. Swathes of Tal'darim were killed, their fleet sent into disarray, and even Amon's own plans were hindered. Nonetheless, he was amused at the bloodshed.
Four years after the Overmind's death, the apparent spirit of Tassadar passed on the creature's vision to Zeratul, who made sure to warn Jim Raynor of Kerrigan's importance and to ensure her survival.
- Narud's final words before returning to the Void.(src)
During the Second Great War, Emil Narud and Valerian Mengsk convinced Jim Raynor to collect a xel'naga device, at first for profit, and later to restore Sarah Kerrigan's humanity. This brought him into conflict with Nyon's rogue Tal'darim faction which guarded most of the pieces, as well as Sarah Kerrigan, who feared it. After collecting all the pieces, Raynor took the completed device to Char and used it to de-infest Sarah Kerrigan, freeing her of Amon's taint.
After returning to the Koprulu Sector, Kerrigan was contacted by a resurrected Alexei Stukov, now an infested terran. He directed her to Skygeirr Platform, which was producing hybrids under the direction of Doctor Emil Narud. Kerrigan and her forces fought their way through Narud's forces, which included Dominion troops, hybrids, and Tal'darim protoss. Eventually she found her way to the xel'naga temple beneath it. There, she battled Narud, who revealed himself as a servant of Amon. Kerrigan defeated him, but in his last moments he revealed that Amon was alive and would be returning soon. After defeating Mengsk on Korhal, Kerrigan departed to do battle with Amon.
Alexei Stukov would later correctly speculate that the xel'naga device used to de-infest Kerrigan may have actually had a hand in reviving Amon. He noted that the original Queen of Blades had tremendous power, and that the device couldn't just simply erase it, surmising that it had to "go somewhere." Kerrigan wondered if hybrids had been on Char during the battle there to collect the energy on hand.
Later, Phasesmith Karax, after extensively studying the keystone, discovered that the device had been modified by Narud to extract the void energies from the Queen of Blades on Char. These energies had been stored within the device, and later used to revive Amon. 
Collusion with Ma'lash
According to Nuroka, once the First Ascendant of the Tal'darim, Amon's true plan involved destroying all creations of the xel'naga, including the Tal'darim. Amon saw death as the highest ideal, not failure. His saying, "Defeat your masters, or fall beneath them" was really "Defeat your masters, or rise beyond them". Nuroka believed that Highlord Ma'lash knew this as well, but Ma'lash continued to support Amon and kept this secret from the Tal'darim. Nuroka said that Amon was pleased when the Tal'darim killed each other, which could happen by the hundreds in Rak'Shir duels.
Determined to save his people, Nuroka enlisted the support of Fourth Ascendant Alarak, intending to kill Ma'lash and reveal the secret to the Tal'darim. In the end, Alarak sided with Ma'lash, and Ma'lash killed Nuroka, allowing Alarak to take Nuroka's place as First Ascendant. However, Alarak now knew the truth and intended to plot against Amon secretly to save his people from obliteration.
Legacy of the Void
Whispers of Oblivion
Zeratul tracked Amon's place of rebirth to Atrias, where he believed he could learn the final piece of the xel'naga prophecy. During his infiltration of the Temple of Erris, the Tal'darim highlord Ma'lash used a Void catalyst to contact Amon. Amon ordered the highlord to prepare the Tal'darim for war. Zeratul witnessed this and, hoping to delay Amon's forces, launched an assault on the catalyst, destroying it. As the device exploded, "Tassadar" appeared in the remains of the catalyst and told Zeratul that "the Keystone shall usher you unto hope". Afterwards, Amon reappeared in the catalyst's wreckage, killed many of Zeratul's troops with his powers and began summoning shadows of the Void, as well as a few hybrids to kill Zeratul. Amon then set the temple to collapse. Praetor Talis and her forces sacrificed themselves to buy Zeratul time to escape. With the last piece of the prophecy in hand, Zeratul set out to warn Artanis.
Corruption of the Khala
Amon made his presence known to the Daelaam during their attempt to reclaim Aiur from the zerg. Hybrid reavers on the surface took control of the feral zerg on the planet's surface, coordinating them into a more dangerous fighting force. However, the reavers were defeated, and the invasion continued. It was here that Amon made his move, as he began corrupting the Khala, taking numerous Khalai under his thrall. This included Hierarch Artanis himself. Zeratul attempted to remove Artanis's nerve appendages (which would cut off his link to the Khala), but Amon would not let his puppet go so easily. Speaking and acting through Artanis, Amon fought Zeratul, gaining the upper hand. In a last ditch effort, Zeratul struck, severing Artanis's nerve appendages, albeit at the cost of his own life.
Now freed of Amon's control, Artanis saved as many protoss as he could, including phase-smith Karax, before deciding to flee Aiur in the Spear of Adun. It didn't take long for Amon to find a new thrall in the form of Executor Selendis. Through his puppet, Amon mocked the Daelaam, and sent his possessed protoss and zerg against them. However, Artanis and his followers managed to activate the Spear of Adun and escape Aiur just in time.
Let the Galaxy Burn
Amon also began building himself a host body on Aiur, using pieces of the Overmind's corpse, as well as many of the enslaved protoss. In the meantime, his servants carried forth his will. The terran Moebius Corps, accompanied by hybrids, carved a path of destruction through the Dominion. Through the zerg of Aiur, he had an army. Furthermore, the Tal'darim remained his servants. The plan he had set into motion millions of years ago was now in its final stages.
The following section contains timeline information which isn't necessarily in chronological order.
Dealing with the Devil
- Amon to Artanis(src)
Amon spoke to Artanis through the Preserver Rohana, who remained within the Khala. Amon defended his actions, claiming that prolonged resistance would only prolong the suffering of his foes' existence. However, Rohana was able to regain control.
Amon sent his brood through the warp gate that linked Shakuras with Aiur. The zerg quickly gained ground, to the point where Talematros was the last bastion. Talematros was successfully evacuated, and Amon observed the protoss take up positions at the planet's xel'naga temple. He sent his zerg forces to test them before unleashing his hybrid. The protoss were able to overload the temple's phase prism and send it into the planet's core. Shakuras was destroyed, and took over a billion of Amon's zerg with it.
Artanis and Kerrigan eventually made their way to Ulnar, though they had to fight their way through Amon's forces. Seeking to find the xel'naga, they found only their corpses. Amon's voice echoed through the chamber, and he granted Artanis a vision of his new host body. He then opened a portal to the Void, as his forces engaged those of Kerrigan and Artanis.
Onboard the Spear of Adun, Amon once again spoke through Rohana, mocking Vorazun. He once again offered release from the cycle, even as Vorazun demanded that he release the preserver. However, despite Amon's threats, both the protoss and zerg were able to escape from Ulnar.
The Return to Aiur
"Amon. I have destroyed your terran army. Eradicated your hybrid facility. And turned your servants to my side. All that is left is your destruction."
"Impudent speck! You are insignificant in the face of the salvation I bring. You will come to Aiur and die."
- Amon and Artanis(src)
After Ulnar, Amon's plans began to go awry. The Daelaam brought the Tal'darim into the fold by First Ascendant Alarak claiming the mantle of highlord. The hybrid production facility at Revanscar was destroyed, and Moebius Corps along with it. Despite the efforts of Amon's zerg, the Daelaam were even able to reactivate the purifiers stored on Cybros. Speaking through Rohana one last time, Amon addressed his foe, making his frustrations plain to see. It also allowed Rohana to gain insight into Amon's plans to use the Golden Armada to his own ends.
The Daelaam arrived at Aiur, destroying the psionic matrix that stemmed from the planet, slowing down the arrival of the Golden Armada. All that was left was to destroy Amon's host body before he fully awakened. However, Amon contacted Artanis again, this time without a host. He showed him that he was too late, that his body was ready. He mocked Artanis, but defiant to the end, Artanis ordered that the attack be launched. Amon was protected by Void shards, to the extent that he was immune to any attack the protoss could launch. Yet despite his efforts, the Void shards were destroyed, leaving Amon exposed. He unleashed his power on his attackers, but could do nothing as the Daelaam fleet unleashed its weaponry from orbit, destroying his body.
Even then, Amon had one last card to play. The Golden Armada was nearing Aiur, and he still remained within the Khala. Using the Keystone, the Daelaam were able to hold off Amon's thralls long enough for the device to activate, temporarily removing him from the Khala and imprisoning him in the Keystone. As Amon struggled to break free from the Keystone, Artanis urged Selendis and the other liberated Templar to sever themselves from the Khala. The remaining Khalai severed their nerve cords, and with them, Amon's connection to his thralls. Thus, he was banished to the Void.
Defeat and Death
"You are but a product of the flawed cycle. Manipulated for the entirety of your existence."
"You know nothing about me! I care little for the xel'naga's Infinite Cycle or your twisted lies. No. I choose something different. I choose freedom...for all of us."
- The final confrontation between Amon and Kerrigan.(src)
Despite his defeat on Aiur, Amon had not ceased his desire to break the Infinite Cycle. Within the Void, he began plotting his return. His strength grew by the moment, and his return seemed inevitable, however distant it might be to the species of the galaxy. Summoned by Ouros, terran, zerg, and protoss forces entered the Void to defeat Amon once and for all. The allied forces made headway, slaying Amon's servant Duran, and freeing Ouros. Ouros transferred his essence to Kerrigan, making her xel'naga herself. In the depths of the Void, the two xel'naga confronted each other. Amon was unfazed—he had killed xel'naga before, and his Void crystals surrounded him with a barrier of energy that made him virtually invincible.
Kerrigan and her allies launched a final assault on Amon, targeting his Void crystals. During the battle, Amon unleashed hundreds of void shades, as well as numerous void constructs against the allied forces. He even resorted to physically ripping the ground out from underneath the allied bases. Despite this, Kerrigan and her allies destroyed enough of Amon's void crystals to render him vulnerable.
Kerrigan confronted Amon, who, defiant and unrepentant, told her she was nothing but a pawn in a flawed cycle and that she had been manipulated her entire life. Kerrigan retorted that she cared neither for the xel'naga's cycle nor Amon's lies, and that she instead chose freedom for all life. She then blasted Amon with a single powerful beam of energy, finally killing the dark god. After his death, rumors ran rampant throughout the Dominion as to his ultimate fate, but only a select few knew the truth, and they remained silent.
Raw data exists for Amon in the map editor.
Personality and Traits
To Amon, death was the highest ideal. For all the adulation the Tal'darim give him, he possessed no love for his subjects, mocking those who took in Rak'Shir. He demonstrated overconfidence and egotism on occasion, taunting his foes when certain of victory. Even when his own plans were set back, he found amusement in watching the Tal'darim slaughter one another. In general, he viewed the whole of the protoss race with disdain and referred to them as "foolish, prideful children." According to Rohana, the only creatures Amon truly cared about were his hybrid.
Amon did not view himself or his actions as evil. He believed that the Infinite Cycle brought unnecessary suffering and was corrupt. He often claimed to his followers and foes alike that by destroying the creations of the xel'naga, he was bringing an end to their suffering. According to Rohana, however, Amon actually desired to destroy life out of an intense hatred for the xel'naga. There is evidence that Amon's resentment towards the xel'naga and the Infinite Cycle is in part due to his transformation into a xel'naga himself, a process which Amon claimed was forced on him.
Amon was willing to sacrifice his own forces if it meant defeating his enemies.
Amon was supposedly immortal and could only be slain by another xel'naga. Even after his apparent death prior to the Great War, he retained enough cognition to observe and even influence events outside the Void. Within the Void, he was said to have matter-manipulation abilities, and that without his blessing, no protoss could survive long there. At the least, he demonstrated great power, able to communicate with others even without a Void conduit, and create and/or dispatch shadows of the Void and Void constructs en masse. He could also summon and control Void thrashers. 
Amon had the ability to transfer his consciousness into any protoss connected to the Khala. Protoss under his thrall in this manner would show visual signs (e.g. red eyes and psi-blades), and speak in a combination of Amon's voice and their own. Only the Grand Preserver Rohana is known to have resisted his control while being connected to the Khala, and even she temporarily fell under his control several times. 
Even when disconnected from the Khala, the protoss were not completely immune to Amon's mental powers. On two separate occasions, Amon spoke to Artanis telepathically, taunting the Hierarch with visions of his host body being constructed on Aiur. In the first instance, Amon levitated Artanis's body while speaking to him. On the second occasion, Amon briefly disrupted power to the Spear of Adun's bridge before revealing his presence. Amon could also communicate with his Tal'darim followers through the substance terrazine, although he could not possess their bodies.
Even among other xel'naga, Amon was incredibly powerful. Prior to his defeat at Ulnar, he had fought and killed many xel'naga. According to Zurvan, Amon could literally rip entire planets apart. Even when weakened, his power within the void was strong enough to imprison the xel'naga Ouros. 
Amon's host form was also quite powerful. Upon awakening on Aiur, he used his new body to raze much of Artanis's base with a single blast of energy. However, his host body also proved vulnerable to conventional weaponry, as seen when an orbital bombardment from the protoss fleet completely incinerated his host form mere moments after it awoke.
Amon was able to resurrect his fallen warriors, granting them new bodies.
- Blizzard has clarified that the Voice in the Darkness is a different entity and not the same thing as Amon (otherwise known as the "Dark Voice").
- One of Amon's quotes is "I love it when a plan comes together", the signature line of Colonel John "Hannibal" Smith of the A-Team.
- "Amon" is a name that exists in real-world myths, specifically Egyptian, Judaic, and in daemonology. He has been likened to the Biblical figure of Lucifer.
- According to Kerrigan's StarCraft II hero page, it was Amon who ordered her to retrieve the xel'naga device. This is in contrast to comments made at BlizzCon 2011, where it was established that Kerrigan sought the device from her own volition, and Heart of the Swarm itself, where Kerrigan claims that Amon never directly controlled her. Paradoxically, such an order would have been in opposition to the actions of Emil Narud (his servant).
Amon was first hinted at in the bonus "Dark Origin" mission of StarCraft: Brood War. As a character, Amon was conceived before work began on StarCraft II. In early readings, Amon came off as being one-note, as the writing team were faced with the reality that while they knew Amon's motivations, they were not so apparent in-game.
Amon appeared as the primary antagonist in Legacy of the Void. In the context of the story, James Waugh has commented that Amon is the thematic inverse of Artanis, that while Artanis seeks to work with and improve upon a flawed world, Amon's reaction is to simply to "shatter it all." A goal of the game was to reveal Amon's humanity within the story.