Revelation 20:1

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

Cross References

Revelation 1:18 I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.
Revelation 10:1 And I saw another mighty angel come down from heaven, clothed with a cloud: and a rainbow was upon his head, and his face was as it were the sun, and his feet as pillars of fire:
Luke 8:31 And they besought him that he would not command them to go out into the deep.
2 Peter 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
Jude 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
Revelation 9:1-2 And the fifth angel sounded, and I saw a star fall from heaven unto the earth: and to him was given the key of the bottomless pit. And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit.
Revelation 18:1 And after these things I saw another angel come down from heaven, having great power; and the earth was lightened with his glory.