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 artifact, you must defeat certain monsters within the Doom Gauntlet
The base chance of achieving an artifact is about one in a thousand (or one in a hundred from Dark Fathers). These odds improve as you make more and more kills until such time as you receive an item, at which point the counter resets.
"For your valor in combating the fallen creature, a special artifact has been bestowed upon you."
Here is a list of the artifacts you can obtain
Here is a list of the items you can get recipes to make.