Abaddon is the angel of the bottomless pit of hell. It is called Abaddon in Hebrew and Apollyon in Greek.
Rev 9:11They had as king over them the angel of the bottomless pit; his name in Hebrew is Abaddon, and in Greek he has the name Apollyon.
In Proverbs, it is mentioned along with Sheol.
Prov 27:20Sheol and Abaddon are never satisfied, and the eyes of man are never satisfied.
Prov 15:11Sheol and Abaddon are open before the Lord; how much more the hearts of men!