Mordegon

Mordegon is the main antagonist of Dragon Quest XI. He's an evil from ancient times that is bent on getting the World Tree's power and destroying the reincarnation of the Luminary.

Biography
Prior to the beginning of the game, he possesses King Carnelian so that he can label the Luminary as the Darkspawn and have his knights hunt him down. He also corrupted Jasper, who was jealous of Hendrix and always felt inferior to him.

He first appears in person when the hero goes to Yggdrasil, stealing his Luminary powers for himself. He crashes the Yggdrasil Tree into Erdrea, resulting in countless deaths. Veronica sacrifices herself to ensure that the others survive.

He then has his minions (led by the Spectral Sentinels, which are themselves led by Jasper) search for survivors so that he can take their souls. He sets up operation at the Fortress Of Fear, which was where the Yggdrasil Tree was.

As a side-effect of Mordegon taking over the world, monsters such as the Gloomnivore are able to take advantage of the situation.

However, the hero eventually defeats him. Afterwards, the hero then goes back in time to prevent the destruction of the Yggdrasil Tree. He kills Jasper to ensure he isn’t discovered, but is defeated by the hero. However, stopping Mordegon ends up allowing another evil to return.

It is eventually revealed that he is in fact the evil side of the mage Morcant, who betrayed Erdwin in an attempt to claim Calasmos' power for himself. Though he still attempts to prevent Calasmos from being resurrected, he attempts to take over the world.

Trivia

 * He is the first Dragon Quest XI villain to kill a playable character.