Demon Lord

The Demon Lord (魔神王, Majin-ō) also known as Dark Lord, is an unseen antagonist in the Goblin Slayer series. Ten years ago, he was defeated by a group of adventurers.

Appearance
Although he has yet to make an official appearance, a flashback depicts the Demon Lord as a gigantic and menacing figure with demonic wings, red eyes as well as a pair of horns on the sides of his head.

Light Novel Volume 2
The Demon Lord is the overarching figure worshiped by the Evil Sect in charge of various monsters and the like, some of which were recruited as his generals. Ten years before the events of the main series, he was defeated by a group of adventurers, which included Sword Maiden.