John saw in the sky a marvelous sign of the seven angels who were responsible for inflicting people with the seven last plagues.

Revelation 15:1

15 Something else very unusual appeared in the sky. I saw seven angels, whose duty it was to punish (OR, inflict hardship upon) rebellious people with seven different plagues (OR, in seven different ways). God is so angry with rebellious people that this is the last time that he will punish them with the purpose of giving them an opportunity to turn away from their sinful behavior.