"The optimization of weapons, armor, and ships is a phase-smith's highest calling, and Karax is one of the most precise engineers in the whole of the Khalai. He rose above his station in the protoss' worker caste to become a crucial asset Artanis's war against Amon, earning the title of templar and causing Artanis to declare the end to the age of castes. As the Phase-smith responsible for restoring the Spear of Adun's systems Karax is intimately familiar with the Arkship's suite of destructive weaponry. During his time with Artanis, Karax also recovered some of the Purifier-based weapon systems which you'll find in his Co-op Mission arsenal."
Karax is a selectable commander in the Co-op Missions mode of Legacy of the Void, added in the 3.1 patch. He utilizes an array of advanced protoss weaponry and Purifier-type units to yield an army of robotic attackers, and has access to the Solar forge, which can be upgraded to enhance the Spear of Adun's abilities. All of Karax's combat units have a 50% increase in their resource costs, but fully upgraded they are very effective. He is overall more of a support commander, using his chrono abilities to improve production times for him and his allies, building static defenses to defend points of control, and utilizing the Spear of Adun's powers to attack from afar. However, he is still perfectly capable of fielding a powerful army if required.
Unlocks upgrades for the immortal and colossus at the robotics bay. • Unlock the immortal's Shadow Cannon ability, which deals 200 damage to an enemy ground or air unit. • Enable the colossus's attacks to set fire to the ground for additional damage over time.
Unlocks upgrades for the mirage and carrier at the fleet beacon. • Allow mirages to become temporarily invulnerable after taking damage. • Allow carriers to automatically repair nearby mechanical units.
The solar forge provides bonus energy to the Spear of Adun, which powers many of Karax's abilities. The first upgrade at the forge will increase the amount of bonus energy supplied, enabling Karax to use the powerful abilities which the Spear of Adun provides more often. Chrono wave is an incredibly powerful ability. Before using it, consider informing one's fellow player of the intention to use it. This will allow them to to start upgrades, queue units, and generally ensure that both of you recieve the massive benefits that the ability is capable of providing. Orbital strike can provide quick support to one's ally.
Karax's strength is about researching technology, aiding partners with spells and deploying static defense: you have the strongest hero powers by far and the strongest static defense by a significant margin. The drawback is that his standing army is pitifully small. Even when only investing in spells and carriers, you'll only have 6~8 carrier(s) at the end of the game; and the strong feeling that Karax was carried by his ally. Because his units cost 50% more, you shouldn't play Karax like you play any other hero. The first 10 minutes of the game, Karax only has "token defense force"; only at the 15 minute mark will production "ramp up".
Swann synergizes perfectly when paired with Karax, due to both commanders' focus on mechanical units. Karax's powerful anti-ground forces with the immortal and colossus are well complemented by Swann's powerful anti-air goliath and thor. Swann's science vessels and Karax's carriers can repair mechanical units, and both can combine their static defenses together for more versatility. Karax is incredibly gas starved, and Swann goes a long way to alleviate this. This is the only way to obtain a true army of powerful units for Karax. Karax's passive heal of mechanical units comes in extremely handy in the early game; Swann's units are as strong as they are slow, making it more difficult for Swann to retreat in battle. Karax's reconstruction beam ability is particularly useful in this case. If all else fails and both players suffer great losses in battle, Karax can help both players recreate their armies faster than anyone else.
Raynor synergizes only decently with Karax; Raynor can produce multiple barracks to quickly rebuild his army and thus won't be helped as much by Chrono Wave. Reconstruction Beam may not affect Raynor's infantry, but it can help his mechanized units and bunkers, the latter of which normally have survivability issues at higher difficulties. Vikings and marines can help Karax compensate for his lackluster anti-air capabilities.
Abathur is has some ability synergies with Karax, as his chrono wave can boost his research and production which is essential to Abathur's army strength. Toxic nests work well with Karax's static defenses, and the Spear of Adun's orbital bombardment abilities can help cover Abathur from air units in the early game.
Kerrigan has access to mutalisks and ultralisks; units that take a long time to create, and would benefit greatly from Karax's chrono-boost ability.
Zagara units are already created quickly due to her relentless swarmer trait, but she can still benefit from Chrono Wave for research and upgrades. Unity Barrier can make her frail units slightly more durable, and Karax can cover for Zagara's lackluster base defenses.
Alarak is a moderately effective teammate for Karax. While Alarak is geared towards overwhelming offense, Karax favors impenetrable defense. Thus Alarak can focus his attention on attacking the enemy while Karax defends their bases and uses his calldown abilities to focus down stronger units Alarak may have trouble with. Reconstruction Beam is invaluable to support Alarak's fragile wrathwalkers and vanguards, and carriers with repair drones makes a powerful army combined with Alarak's robotic forces. The player can use Alarak's Structure Overcharge to power up Karax's defenses even further when faced with an overwhelming enemy force.
Artanis lacks synergy with Karax, as Artanis's army is simply better than his and at a lower cost. Artanis has dragoons while Karax lacks a ranged counterpart, his tempests beat carriers at damage, and reavers can out-burst colossi. Additionally, Artanis's warp-in structures use charges, so Chrono Wave isn't particularly helpful in rebuilding an army if he has a sufficient number of structures. However, Guardian Shell can help by giving Reconstruction Beam time to heal up critically damaged units in a fight.
Vorazun benefits from Chrono Wave if she is building an air force or rebuilding an army, as dark templars and dark archons have a long cooldown on warp gates. She also benefits from Chrono Wave in her tech, as she needs to research many upgrades to reach full power. Her Vortex can also bunch up the enemy for Karax's Orbital Strikes and Solar Lances. Her Dark Pylon can also cloak Karax's defensive structures, making them much more effective.