Revelation 5:2
And I saw a mighty angel proclaiming in a loud voice, "Who deserves to break the seals on it and open the book?" [Note: This "book" was probably a rolled up scroll written on both sides, with wax seals keeping it from unrolling and its contents from being exposed].
Revelation 5:5
Then one of the [twenty-four] elders said to me, "Do not cry; look, the Lion from the tribe of Judah, the Root of King David, has gained the victory, so He can break the seven seals on the book and open it."
Revelation 18:21
Then a mighty angel picked up a rock the size of a huge millstone and threw it into the ocean, saying, "This is how the great city of Babylon will be violently thrown down and will never be found again.
Revelation 10:1
And I saw another mighty angel coming down from heaven. He was covered by a cloud and there was a rainbow above his head. His face was like the sun and his legs were like pillars of fire.