They were ruled by a king, the angel of the Abyss. His name in Hebrew is Abaddon, and in Greek it is Apollyon.
KJV
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.
ASV
They have over them as king the angel of the abyss: his name in Hebrew is Abaddon, and in the Greek tongue he hath the name Apollyon.