ToME WikiMount Doom is a special dungeon that branches off from Mordor level 65 (one level short of the bottom; look for purple staircases). The entire dungeon has a fire theme - floors are all lava, walls are "lava walls", and fire monsters abound. So bring fire immunity, of course! One possible source is Narya from the Dimensional Gates special level. If you don't have immunity, then you can still rest on doors, stairs, and the like, though everything in the dungeon periodically suffers fire damage, whether on a safe square or no. Mount Doom is one of those dungeons that forces you to walk the long way down - you can't use Thunderlord teleportation or the level-teleporting ability of Probability Travel. There's little point in sticking around; while the monsters aren't especially dangerous if you have fire immunity, the annoyance of getting messaged every time an item resists being burnt up is high.
At the bottom of Mount Doom (level 99), you'll find the confusingly-named Mt Doom special level.