Really cool ball cap with Satan's Drano embroidery on the front and woven label on the inside. One size fits all.
The obsidian warrior helm is part of the untradeable and degradable Obsidian armor set. It requires 60 Defense to wear, and can be made by smithing two obsidian bars, requiring 82 Smithing.
Obsidian bars are made by smelting obsidian shards, which in turn are dropped by monsters in the Fight Cauldron.