Revelation chapter 9 contains one of the most frightening and terrifying chapters in Scripture.  It describes two diabolic armies that will be divinely unleashed on the earth during the Tribulation.  This 7-year period of time (Daniel's 70th Week) is not only God-orchestrated to purge and prepare Israel to accept her true Messiah and to bring many Gentiles to saving faith, but to punish the ungodly - those who willfully refuse to repent and come to Christ.

The fifth trumpet judgment (which is also the first of three "Woe" judgments) brings about the dreadful army of demonic locusts that will be loosed from the bottomless pit.

There is a lot of discussion about fallen angels and confined fallen angels in this lesson.  There is also an interesting discussion on the identity of "Apollyon" (Greek) or "Abaddon" (Hebrew), the king of the demonic locust army.