Revelation 9:11

They were ruled by a king, the angel of the Abyss. His name in Hebrew is Abaddon, and in Greek it is Apollyon.

Note Abaddon means Destruction.

Note Apollyon means Destroyer.

BSB · Berean Standard Bible
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.
KJV · King James Version
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”.

Note “Abaddon” is a Hebrew word that means “ruin”, “destruction”, or “the place of destruction”

Note “Apollyon” means “Destroyer”.

WEB · World English Bible
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.
ASV · American Standard Version

Related passages

From the Treasury of Scripture Knowledge, grouped by the word they explain.

they had
Revelation 12:9; John 12:31; John 14:30; John 16:11; 2 Corinthians 4:4; Ephesians 2:2; 1 John 4:4; 1 John 5:19
the angel
Revelation 9:1
Abaddon
that is, a destroyer.
John 8:44