Rev 9:11

11 They have over them as king the angel of the abyss. His name in Hebrew is “Abaddon,” but in Greek, he has the name “Apollyon.” 

They have as king over them, the angel of the abyss – Satan, who was thrown into the abyss, for one thousand years, in Revelation 20:3.

Abaddon, Apollyon – in Greek, Apolluón, meaning “destroyer.” The Antichrist who comes to “kill and destroy,” in Greek, apollumi, (John 10:10). As contrasted with Christ, who came to bring “life and life abundantly.”