Archon is a Mastery Wizard skill unlocked at Level 30, which transforms the Wizard into a being of pure energy. While in Archon form, the Wizard has access to a powerful set of unique Archon skills.
Archon form is the most damaging spells and attacks possible for the Wizard. The DPS output is massively increased and it's a great way to transform and take on Elites in the higher difficulty levels. It is not exactly a panic button though, in the way that the Barbarian's Wrath of the Berserker transformation is, since the primary boosts for the Wizard are offensive, not defensive. You can quite easily die while in Archon form, and the limited skills available decrease the Wizard's strategic options considerably.
The following abilities are only available while in Archon form. Other normal Wizard skills can *not* be used while in Archon, so there is a trade off to this form; much higher damage, but with a limited variety of offensive skills, and little or no defensive/movement skills.
The skill was first available to players in the BlizzCon 2011 PvP Arena demo. The Eliminator talked about some of the Archon skills in his PvP demo report:
The most popular skill in Archon form was the Beam. It worked just like a much more damaging Disintegrate, though it had a shorter range. I don’t know if the skill does critical hits, because I can’t remember ever seeing any when I used it. The beam did do a low of 900 to a high 1300 a second. That was without things being hit by Arcane Torrent or being Frozen by Frost Nova. If they had either debuff on them they died fast.
The Eliminator also shared some general advice:
It only took one encounter with an Archon to know you don’t want to mess with a healthy one solo. One that was at about half health would go down rather fast from a WoTB Barb and a few Hammers. An Archon Wizard really felt like a game changer, but they weren’t unbeatable if you dealt with them quickly, and since the Wizard does not have Teleport in Archon form, they could be avoided as well.
That demo presented a souped up version of Archon, with Slow Time and Teleport both available. In the release client, these are only enabled by rune effects, and players can not have both at once, forcing a more discrete choice between defensive or movement capabilities.
The Diablo III Beta started in September 2011, and while it ran, all of the skills continued to undergo extensive iteration. Skill changes varied from changing the levels the skills are unlocked, to changes in resource cost, damage, and even function. A few skills were removed altogether, and a few new ones were added. Also, the entire skill system received an overhaul with the Beta Patch 13 update in February 2012, with physical runes replaced by rune effects that automatically unlock at various levels for the different skills.
With the Patch 13 changes, Archon was classified as a Mastery skill and moved to level 30.
A video showing the massive potential of this ability when paired with great equipment can be seen below. This video is somewhat deceptive as it only shows a few excerpts of locations when Archon is most effective; against slow enemies, without ranged attackers, when there are numerous targets to keep refreshing the duration, etc. But it's still an amazing killing spree for shortly after release, when Inferno was still unbalanced and too difficult for most players to survive at all.