It is one of the rarest set items in the game due to being a TC 3 base item with a comparatively high qlvl of 20. Beyond Normal Act 1, most monsters only have a small chance to drop from TC 3, and due to the qlvl, only monsters with level 20 or higher can drop it. This means it can't drop before Lost City in Act 2, where it can only drop from unique monsters. Its rarity is further compounded by being one of two set Caps with Infernal Cranium.
Its set is fairly weak but has other fairly powerful pieces, so it may be incidentally worn with Sander's Taboo or Sander's Riprap.