The entrance to the Fire Caves on Bajor in 2375
The Fire Caves on Bajor are a prison created by the Prophets to contain the Pah-wraiths. Keiko O'Brien visited the caves during an expedition in 2373, and unwillingly became host to one of the Pah-wraiths until it could be forced out by her husband Miles. (DS9: "The Assignment")
In late 2375, Kai Winn Adami and Dukat attempted to free the Pah-wraiths from the caves, using the Book of the Kosst Amojan. One of the wraiths did escape and joined with Dukat who then prepared to release the others, killing Winn in the process, but Benjamin Sisko was able to throw himself, the book and Dukat in to the fire below, trapping Dukat with the Pah-wraiths forever. (DS9: "What You Leave Behind")