Also known as Apollyon.
Abaddon literally translates from Hebrew into destruction, ruin, and perdition. This name is also used to refer to the place of the dead. Apollyon, in the Old Testament, means Hell.
In Biblical legend, a being regarded as the angel of the bottomless pit, described as so in Revelation 9:11:
And they had a king over them, which is the angel of the bottomless pit, whose name in the Hebrew tongue is Abaddon, but in the Greek tongue hath his name Apollyon.
There, he is described as the king of a group of locust-like creatures with the faces of men, the hair of women, tails of scorpions, teeth of lions, wings that sound like the thunder of horses' hooves charging into battle and huge claws. He is the chief of demons of the seventh heirarchy.
The fifth angel of Apocalyptic tradition will open the bottomless pit, releasing these creatures that will, for five months, torment people that do not have the seal of God upon their foreheads. After these torments are complete Apollyon is supposed to sieze Satan himself, bind him, and toss him into the bottomless pit for one thousand years.
Cornelius Agrippa was the first person to equate Abaddon with Apollyon, positioning him as monarch in the lower shadow of the sphere in the planet Venus.
Apollyon's role in the bible is unsure. Sometimes he is portrayed as a good angel who assists God, other times he is seen as a fallen angel succumbed to evil.