From UO Excelsior Wiki - Ultima Online Free Shard
The Dungeon of Doom is the source of some of the most powerful Artifacts around. They all have the Artifact Rarity property with a value of 11, with the exception of the Legacy of the Dread Lord and The Taskmaster (which are only at level 10); this is a fairly abstract figure, as there is no set rate at which the items enter the game world. There also exists a set of Doom Stealables (some of which are in very short supply and hence also very valuable).
To acquire a standard Doom arti, you must defeat certain monsters within the Doom Gauntlet:
They are not looted, but instead placed directly in your Backpack (or Bank Box, should you be overweight) with the following system message:
"For your valor in combating the fallen creature, a special artifact has been bestowed upon you."