A katagon bar is a bar of refined Katagon ore. It can be created through the Smithing skill by using katagon ore on a furnace, requiring level 70 Smithing and granting 51.1 Smithing experience. The katagon ore itself can be mined from the rocks scattered throughout the dungeon, obtained as a monster drop, or bought from the Smuggler.
A katagon bar can be smithed on an anvil through the Smithing skill to create Katagon equipment, granting between 73 and 77.5 Smithing experience per bar used. A list of katagon items that can be smithed from katagon bars and the Smithing levels required can be found here.