"While High Templar and Dark Templar are undoubtedly forces to be reckoned with, there is a truly magnificent warrior that stands, quite literally, above them both. When two Templar of any kind combine, they have the ability to form what is known as an Archon. It should come as no surprise that the source of an Archon's overwhelming power, or the nature of its formation, have been a mystery to the public. Now however, as unbelievable as it might seem, we have quite literally had this information fall from the sky and land into our hands. We now understand that an Archon's physical form shapes and anchors the psionic maelstrom swirling around it, and that without this form, the Archon would burn out like a star immediately after merging. We will tell you all about this and more in the full report which you can find here."
"In times of great need, two High Templar or Dark Templar can merge into a single entity that radiates overwhelming psionic power and rage. The decision to lay down the lives of valuable Templar is never made lightly. Perhaps their energy was spent. Maybe the warriors themselves chose this final act of sacrifice over succumbing to their injuries. Regardless of its origins, the Archon's presence on the battlefield fills the minds of all beings - psionic or otherwise - with primal fear and awe."
Archons are beings of psionic power and rage, and their presence on the battlefield inspires much fear in their foes. The creation of an archon is a serious affair and is never made lightly, for it requires the reluctant sacrifice of valuable Templars for a short-term gain. Archons radiate energy like stars and burn out when depleted.  The archon is, for the most part, a being of pure energy that hovers above ground by displacing gravity itself. Nonetheless, this embodiment of power has a nigh-intangible physical form with unmistakable protoss features; a remnant of the Templar who sacrificed their lives to bring it into being. Its purpose is to shape and anchor the psionic maelstrom, preventing the archon from burning out like a star as soon as the merging is complete. The merging results in the loss of self for the Templar who create the archon, and those who do so are bequeathed great honor and a place in the templar archives.
The loss of most of the psionic powers of the two merging templar is more than compensated for by the archon's formidable psychic gestalt. Archons can independently erect a resilient shield and direct psionic shockwaves. Emotionally, archons are manifestations of pure rage, though this may be "twisted" should the merging be in error. Archons are much feared by their enemies, and can wade through countless enemy troops.
Originally, archons were formed through the merging of two high templar. After the Brood War and the reunification of the protoss, archons of a different kind can now be created by merging two Dark Templar or even a Khalai and Nerazim merging together.