Eidolon is the third, final, oldest and most powerful of the three Serpent Riders. Whilst D'Sparil and Korax attacked and took control of Parthoris and Cronos, respectively, Eidolon took control of the world of Thyrion.
Eidolon is fought at the end of Hexen II as the final boss of the game. At first, Eidolon confronts the player by himself, but when the player proves to be worthy, the Serpent Rider calls upon the Chaos Sphere to increase his size three fold.
In the final hub it's revealed that Eidolon has the power of life and death.
In his first form, Eidolon will follow the player whilst launching balls of lightning that home and bounce towards the player. The key to defeating Eidolon's first form is to move as little as possible since he predicts player movements. If the player has saved up plenty of Tomes of Power and Kraters of Might, then this battle can be much easier.
Homing Lightning Balls Eidolon launches around five balls of lightning that home and bounce towards the player.
Repulsion disc if you get close enough
The key to fighting Eidolon now is the opposite: move as much as you can but keep an eye on Eidolon and avoid the center area as you can get killed instantly by lighting.
Stomping Eidolon can stomp the player when walking.
Fireballs Eidolon launches a burst of fireballs towards the player. These projectiles move very quickly and hit for a lot of damage.
Fire breath If you get close to Eidolon he'll try to toast the player with hell-fire breath.
Morph Eidolon some times shoots red projectiles that morph the player into a sheep on impact.
Unholy lighting Eidolon's deadliest attack is to summon lighting that instant kills anything no matter how much HP the target has.
Regeneration Eidolon can regenerate as long as the Chaos Sphere is still around. These two are his most powerful spells and represent his control over life and death.