Revelation chapter 20 is a continuation of chapter 19. After the battle of Armageddon is concluded and the beast and the false prophet are thrown into the lake of fire and brimstone, the devil is chained and thrown into the bottomless pit for 1000 years.
“And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand. And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years, And cast him into the bottomless pit, and shut him up, and set a seal upon him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season.” – Rev. 20:1-3