Alastor ("avenger") in Greek mythology, was the personification of familial feuds. He was also associated with sins that pass down from parent to child. As a genius, or spirit of the household in Roman mythology, he incited people to murder and other sins. He was originally a mortal, son of Neleus, King of Pylos. He was later downgraded to a minor demon after he and his brothers were killed by Heracles.

In Christian demonology, Alastor became chief executioner to the monarch of Hell. He was likened to Nemesis and the destroying angel and was known as The Executioner. He was exceptionally cruel.

Evil spirits were also known as Alastors.

Devil May Cry and Viewtiful Joe

Alastor is also a Sword in Devil May Cry, and a winged demon in Viewtiful Joe; both are games created by Capcom. His first appearance in Devil May Cry is a scene where he stabs the protagonist of the game right through his chest. In the game, Alastor has his own will, but is overpowered by Dante, and becomes his standard sword for the most part of the game. In Viewtiful Joe, he serves as a boss in the game, and also as an unlockable character that is playable. He has, just like Dante, a Devil Trigger, and is very reminiscent of Devil May Cry in the way he plays.

Castlevania

Alastor has also appeared as an enemy in the famous Konami series, Castlevania, specifically in Castlevania: Aria of Sorrow. He appears as an translucent silhouette of a Demon with its only visible feature being a broken dark sword. Earning Alastor's soul in that game grants the user the summoning of Alastor's Sword as a Familiar that follows the user and attacks in tandem.

Alastor has also appeared as the leader of Hell's army in Painkiller (game), and the final boss in Battle out of Hell, the first Painkiller addon. He appears as a large, fat, winged demon in Painkiller, and a heavily armoured version of said demon in Battle out of Hell.