The Archon is an enemy Alien unit. It is the new evolution of the Floater, but is now much more polished and is now of a higher rank among alien troops.
Archons are of decent durability, with 12-24 HP, difficulty dependent. They are armed with a powerful staff, able to strike hard (6-9 damage) and fire a plasma beam (7-8 damage). They can fly and have a notably high dodge of 25, making them tough to take down. In addition, if they take damage they will use an ability called Battle Frenzy, giving them an extra action that will almost certainly allow them to close with you next round.
Archons have a third attack, the ability 'Blazing Pinions'. They will fly to the top level of the map and release a volley of missiles which will impact on the next round no matter if the Archon is killed or not, with strike locations being marked on the map. This is a very dangerous attack, not merely for the 4-7 damage dealt, but for it's huge area of effect. Each missile explodes in a 3x3 square, destroying huge areas of cover. Moving all soldiers away from the marker is advised.
The Archon autopsy unlocks the Fusion Blade, the highest tier of swords. Those with the Alien Hunters DLC also unlock the Fusion Axe, the final upgrade to the Hunter's Axes.