1 The fifth angel sounded, and I saw a star from the sky fallen to the earth. The key to the pit of the abyss was given to him.
A star from heaven which had fallen – like Satan in Isa 14:12, and Rev 12:13, an Angel who has fallen from heaven.
the key of the bottomless pit was given to him – in the fifth trumpet, 1/3 of the lawless in the kingdom of heaven are not effected, telling us that the fourth trumpet completed the cleansing of the kingdom, after 2,300 days, as prophesied by Daniel. This ends the 1,000 years in which Satan is bound, as described in Rev 20:2. The key to the bottomless pit is given to release Satan, and his demons, as described in Rev 20:7-8. Satan will ultimately deceive the nations to gather for war at the Battle of Armageddon.