Thanatos

Official Text
Thanatos, also known as the Ascendant God, is one of the many Gods of the Xenia Continent. He is the boss of the final dungeon of the Xenia Continent, the Fortress of Ascension.

In the journey of Battle for Bermesiah, a voice calls the Chase for help, because an even greater evil approaches.

During the events on Silver Land, a Guardian Ent was freed by the Grand Chase, who warns them about an even "darkness" coming. Eventually, it led down to one man: Victor, who was the mastermind of the Silver Land's corruption, and one of Thanatos' minions. Victor is slain, and he asks Thanatos for help. However, a voice tells him that Victor was just his tool and he outlived his purpose, and then leaves him to die.

Upon reaching the continent of Xenia, they learn that this "Ascendant God" grew in immensive power and broke the "balance" between all the deities, corrupting them or causing them to flee. The other deities were freed from possession and then supported the Chase to defeat Thanatos.

When they reach his lair, he is confronted with such power. He may seem frightful, but he was defeated. However, Ashtaroth, one of his loyal guardians, took his essence and left (which results the Chase letting him go, but then turn up to chase him instead). Although Thanatos' corruption stopped, his power is in the hands of another mere being. Who knows what will happen next?

Fan Made
During Erinn's prehistorical time, it was discovered he was actually the brother of Kaze'aze and the son of Cichol. He was fought and defeated in Erinn's End by the God of Twilight.

It is later revealed he was the one who created the Prime Knights, though they turned against him alongside with the Twilight's Blade slaughtering him.

He also put up a peace treaty with Torusice, stating he would leave their lands untouched if he was given access to their Lancers. However, he never used the soldiers, but attacked Torusice, killing all but TwO of them.