The black isle is a isle overflowing with the bones of countless slain heroes lured in by an angry fallen hero trapped at the bottom of the bitterblack dunegon. Unspeakable horrors can be found on the isle, which is far worse than any creatures you can encounter anywhere else in the game. They're often dangerous enough that even if you're able to fight dragons, they will devor you.
What sets the black isles apart from all other dunegons and locations in the game is the general mood of the isles. It's always night there, and various creatures are always going bump in the dark... Often eating eachother alive. The deeper you go into the isle, the more bones of slain heroes you encounter until you're literally swimming across an ocean of blood and bones to try to undo the curse of the isle! After finally overcoming the horrors, you'll come across the fallen hero's forgetten hometown, which is quite an experience to explore.