Abaddon or Apollyon

The Complete Word Study Dictionary: New Testament 12. ἄβυσσος ábussos

12. ἄβυσσος ábussos; gen. abússou, fem. noun from a (1), an intens., and buthós (1037), deep. Abyss, an extremely deep place. It occurs only twice outside the book of Revelation (Rom. 10:7, simply the abode of the dead; Luke 8:31, the prison destined for evil spirits). In Rev. 9:1, 2; 11:7; 17:8; 20:1, 3, it is a prison in which evil powers are confined and out of which they can at times be let loose.

Revelation 9:11 KJV 1900

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.

Revelation 11:7 KJV 1900

And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them, and kill them.

Luke 8:31 KJV 1900

And they besought him that he would not command them to go out into the deep.

Revelation 17:8 KJV 1900

The beast that thou sawest was, and is not; and shall ascend out of the bottomless pit, and go into perdition: and they that dwell on the earth shall wonder, whose names were not written in the book of life from the foundation of the world, when they behold the beast that was, and is not, and yet is.

Revelation 20:1 KJV 1900

And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand.

Revelation 20:3 KJV 1900

And cast him into the bottomless pit, and shut him up, and set a seal upon him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season.

Revelation 6:8 KJV 1900

And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth.

Abbadon or Apollyon rising from the Pit.

12. ἄβυσσος ábussos; gen . abússou, fem. noun from a (1), an intens., and buthós (1037), deep. Abyss, an extremely deep place. It occurs only twice outside the book of Revelation (Rom. 10:7, simply the abode of the dead; Luke 8:31, the prison destined for evil spirits). In Rev. 9:1, 2; 11:7; 17:8; 20:1, 3, it is a prison in which evil powers are confined and out of which they can at times be let loose.

3. Ἀβαδδών Abaddṓn; masc . noun transliterated from the Hebr . ʾAbaddōn (11). Wound, destruction. The Hebr. ʾAbaddōn designates the abode of unbelievers following death and is synonymous with Sheʾōl (7585) which is expressed by destruction and loss (Sept.: Job 21:30; 31:12). In Job 26:6; Prov. 15:11; 27:20 it occurs in conjunction with Sheol; in Ps. 88:11 mistakenly with the grave. Hádēs (86) or Sheol was the place of the departed spirits, while the grave (táphos [5028]; mnēmeíon [3419]) was the depository of the dead body. In Job 28:22, it is linked with death and personified in the same way as death in Dan. 4:23; Rev. 6:8, where death and Hades are companions.

In the NT found only in Rev. 9:11 where it is used as the proper name of the prince of the infernal regions and explained by the word Apollúōn (623), destroyer.1