‘And the fifth poured out his bowl on the throne of the beast, and his kingdom was darkened, and they gnawed their tongues for pain, and they blasphemed the God of Heaven because of their pains and their sores and they repented not of their works.'
‘On the throne of the Beast.' The throne of Satan is mentioned in the letter to Pergamum (Revelation 2:13). It was the source of danger for God's people. Alternately Rome itself may be in John's mind, for the emperors had many thrones, and when persecution was in progress, they would all be seen as the throne of Satan. But in the end what is being referred to is any place which is central to anti-Christ, any place where Satan reigns. They stand up against God and will receive their due reward. The darkening of the kingdom, and the resulting pains, have in mind the swarms of locusts of the fifth trumpet (Revelation 9:2) (compare also Exodus 10:21). Here their work is centralised on the centre of godless rule. But the spiritual unrest and anguish they cause do not bring about repentance, rather they result in blasphemy. In the end whatever men sow they reap.
Note how this contrasts with the scenes and result in Revelation 11:9-13. There the earth-dwellers were partying and making merry because God's people had been humiliated and destroyed. So not all are directly included within the effects of the bowl. Nevertheless all will face the final judgment. The description may or may not be seen as including physical plagues. Men in John's day, and through the ages, have suffered from such great sore-producing plagues (compare Exodus 9:9-11). Each time they symbolise God's judgment on those who follow the anti-Christ in whatever form.