Every protoss has a level of psionic capability that is extremely rare among terrans. The protoss of the Templar caste devote their long lives to honing their powerful abilities for war. Of these dedicated individuals, only the most adept and disciplined in the ways of the Khala can hope to eventually rise to the honored rank of high templar. The terrifying power of the high templar is rightly feared by the enemies of the protoss. A mere handful of these potent individuals can lay waste to entire armies of foes.
In times of great peril, two High Templar can take the final step down the Warrior's Path and merge themselves into the awe-inspiring Archon. By abandoning their physical form, the High Templar open themselves up completely to the Protoss gestalt, and they transform into a personification of pure rage. Protected by an immense energy shield, the fiery form of the Archon can wade through countless enemy troops, destroying them with massive bolts of raw Psionic energy. Those Templar that make this final sacrifice are honored forever in the scriptures of the Templar Archives.
These swirling, burning effigies of the protoss spirit radiate incalculable power. Their devastating psionic storms can be unleashed against cowering enemy forces both in the air and on the ground. With the halves of the protoss race reunited, an archon can now be brought into being by the merge of two high templar, or two dark templar, or even the unification of a dark templar and a high templar. Although the protoss are loath to sacrifice valuable templar, those who do achieve this final level of commitment are forever honored in the memories of their people.
The Archon was not initially in StarCraft 2, but the somewhat similar Twilight Archon was instead. The Twilight Archon was an amalgam of the Archon and Dark Archon; a unit with both a strong attack and special abilities such as Feedback. The Archon has returned to the game, replacing the Twilight Archon in the current version of the game.