The Archon is a powerful Protoss unit formed when two High Templar decide to become an Archon by merging. This merging process does not require any other buildings or upgrades. The High Templar, however, does require the Templar Archives and Gateway. Archons use a powerful short-ranged attack that can target both ground and air units, dealing splash damage during the attack. Archons have large hit points due to its robust Plasma Shield and deals potent damage, so despite their short attack range they can devastate enemy infantry forces. Archons work very effectively with Zealots, with the two capable of engaging opponent units simultaneously -- the Archon as a range unit and the Zealot as a melee unit. Archons move fairly quickly, and thus work well with speed-upgraded Zealot and Dragoon land forces. One of the only disadvantages of Archon production is their extremely high cost, given the 300 Vespene Gas requirement.
Archons are usually considered less useful than High Templar Psionic Storms, although Archons and Psi Storms serve a similar purpose in dealing significant damage to units in a small area. Archons are commonly built when High Templar are depleted of Energy. Thus, the Protoss player will rather merge two High Templars to gain immediate use of an attacking Archon to reinforce other troops than wait for the High Templars to replenish their Energy supply. In scenarios where the potent Archons are preferred over the Psi Storm, however, the Protoss player will opt to merge High Templars immediately after they emerge from the Gateway.
The Archon's splash damage does not damage the Protoss player's other units. This contrasts with the splash damage dealt by Siege Tanks or a Spider Mines, which will also damage friendly units if they are within the splash radius. The Archon's splash damage will, however, affect the units of allied players in team games. Beyond the potent damage dealt per second dealt, Archons have no other abilities.
It is notable that the health of the Archons is almost entirely composed of Plasma Shields with very few hit points. Archons can be used in conjunction with Shield Batteries to further extend their life spans. Because of their reliance on Shields, however, the Archon is vulnerable to and thus countered by the Science Vessels' EMP Shockwave.
Archons are primarily used as support units during the early PvZ match-up. Paired with Zealots (as detailed above), they create a very effective combo against both Zerglings and Hydralisks. Archons are not optimal for attacking Sunken Colonies, as they are bulky and not as efficient as Zealots in surrounding the structure. Archons are highly effective against concentrated groups of Zerglings and Mutalisks. Archons are effective for countering Mutalisk harassment since the Zerg opponent will often group and stack the Mutalisks. It is noted, however, that strong Mutalisk micro can counter Archons by repeatedly attacking and retreating, while evading Archon damage. Alternately, the Zerg opponent may separate a group of Mutalisks rather than stacking them before attacking the Archon. The splash damage of Archons can also be useful against a lone burrowed Lurker by targeting a Protoss unit positioned near the Lurker with friendly-fire; this is particularly useful when Observers are not present.
The Archon is very useful in the late game, such as the popular Zerg combination of Ultralisks and Zerglings. Notably, a fully or +2 attack upgraded Archon can kill a Zergling in one hit, regardless of the Zergling's upgrades. Archons are effective against Ultralisks/Zerglings in combination with other units, as their high damage and splash attack make them highly potent. Zealots are often used to absorb damage while Archons deal ranged damage on Zerg armies.
In addition, Archons are important against Defilers as Archons can deal a quarter of their direct damage to unburrowed units, as well as their Splash Damage under Dark Swarm to all units, unlike most other range attack units. Units located at the point of impact of the Archon attack shifted by the Dark Swarm will take full damage. Also, Archons are not significantly affected by Plague as their health is mostly composed of the Plasma Shield.
During long games, the Protoss player will often accumulate a large supply of Vespene Gas as its infantry units are not as Gas-intensive as the Zerg forces, making it easier to build a strong force of Archons.
Archons, while powerful, are not a staple unit in this match-up. Archons have high total health, but it is composed mostly of Shields which takes full damage from both Vultures and Siege Tanks. With the high Gas cost and short ranged attack, the Archon is not as cost-effective as infantry units like the Zealot or Dragoon. Thus, most Archons made in this match-up are due to depleted High Templars in the middle of a battle who have consumed all their Energy casting Psi Storms.
Nonetheless, Archons are not without use. They can quickly deal a large amount of damage to clumped units, such as a Bio Terran army, due to its splash damage and high attack power. It is also noted that Archons are effective in destroying unprotected Spider Mine fields (with an Observer in tow) without risk due to being a hovering unit.
Archons are a strong counter to Zealot heavy armies, since their splash damage is very effective against clumped Zealots. Nonetheless, it is generally advisable for the Protoss player to not merge all available High Templars into Archons as Psi Storms are very effective in the midst of battle.