Demon Lord

The Demon Lord (魔神王), also referred as Demon God, is an antagonist in the Goblin Slayer series, who is the ruler of the demons.

Appearance
In the anime, the Demon Lord is a massive humanoid with purple flames coming out of his skull-shaped head. He is seen with a pair of shoulder plates coupled with a large black cape. A flashback depicts the Demon Lord as a gigantic and menacing creature with demonic wings, red eyes as well as a pair of horns on the sides of his head.

In the Goblin Slayer Side Story II: Dai Katana manga, the Demon Lord is a large dark humanoid who wields a long red sword.

Background
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. It's also iterated throughout series that several Demon Lords existed.

Goblin Slayer Volume 1
The Demon Lord was resurrected a decade after being killed by Sword Maiden and other adventurers, but he was later killed again by Hero.