And the fifth angel poured out his vial upon the seat of the beast; and his kingdom was full of darkness; and they gnawed their tongues for pain, 11 And blasphemed the God of heaven because of their pains and their sores, and repented not of their deeds. Rev 16:10-11 (KJV)
The darkness only seems to occur on the kingdom of the Antichrist. This will be Rome or Babylon. I’m believing that it’s literally Babylon, which is where the Antichrist will have built his kingdom capitol. The darkness somehow causes physical pain.
In many ways, this is a taste of Hell, a place of literal unending fire and torment, in complete darkness.