They are randomly earned when defeating Named Monsters that reside in the nine Mondain's Legacy Dungeons, and appear directly in a player character's backpack. Sentinel Spider inside the Underworld also drops these artifacts.
Alternatively, Peerless bosses will sometimes drop these artifacts on their corpses. In these cases the items must be looted, but sometimes differ from the pack-dropped versions. For example, the Peerless Fey Leggings may lack the Elves Only property.
As these items have been spawning continuously in-game for over two years, they are readily had from vendors and often at very low prices. There are two exceptions to this general rule: the Totem of the Void and the Soul Seeker. The former is valued by mages, Paladins, and Necromancers alike. The latter is a Super Slayer weapon with significant leech modifications and a coveted Special Move, Whirlwind Attack.