One of the main reasons to enter Dungeon Doom can be found in a room to the southeast of the entrance. In the corner of that room lives an ancient sorceress by the name of Victoria, who has been cursed to wander the unholy halls for all eternity.
Victoria has an interesting tale to tell about a creature she calls the Dark Father and also someone called Chyloth the Ferryman. Apparently she once tried to kill the Dark Father, but failed and became cursed instead. To get her revenge for that she will offer to help you get across the lake to the area known as the Gauntlet, where the Guardians keep the Doom Champions.
To receive her help you will however first need to collect 1000 Daemon Bones, which can be found on all creatures in the dungeon. The more powerful the creature, the more bones it will have.
Once you have given the 1000 Daemon Bones to Victoria she will summon a powerful Bone Daemon for you to defeat. If you manage to dispose of the Daemon, searching its corpse will yield a strange golden skull, which will be your ticket across the lake.
Now that you have a golden skull, head for the beach at the lake in the southern end of the dungeon. When you get there you should find a dark platform with a large bell hanging above it. This is the Bell of the Dead and once rung it will summon a mysterious figure known as Chyloth the Ferryman.
Chyloth will yell at you a bit at first, something about only the dead may cross, but if you hand him the golden skull, he suddenly changes his mind and lets you and everyone in your party hop aboard his ferry and sail across the lake.
Finally you are at the infamous Gauntlet of Doom. A place of untold riches and an untold number of dead adventurers. The first thing you should do once you get here, is familiarize yourself with Nix the Variety Dealer. Nix can be found by walking through a wall in the eastern side of the hallway leading south from the beach, there he has made a small shop where he sells all manner of supplies, that could be useful to the brave adventurer.
Further down the hallway you will reach a large hall, this is the center of the Gauntlet. The Gauntlet is made of seven rooms, six of which may contain a Doom Guardian. The seventh room contains a healer, but be prepared to pay, as he charges 5000 gold pieces to resurrect you.
Each Doom Guardian randomly spawns accepting only one type of Doom Token. You will want to be prepared and bring multiples of each Doom Token, or you may leave early, having cleared only part of the Gauntlet.