While it's not explicitly stated as having 99 floors, it is described as being an endless expanse of darkness and fire, making it fitting to be associated with such a number.
However, there's no single dungeon in any game or mythos that definitively has 99 floors of darkness. It's possible you're thinking of a specific work of fiction or video game, but without further context, it's difficult to pinpoint.
Could you provide any more details about the dungeon you're thinking of? For example:
* What game or media is it from?
* What is the purpose of the dungeon?
* Are there any other notable features besides darkness?
With more information, I might be able to help you identify the dungeon you're looking for.