Oblivion Bars are required in the crafting of the Cisgemhibyashoo Bar, the Elemental Excalibur, and the Oblivion Greatsword. The requisite ore for this bar is Oblivion Ore, and it will cost 6 ore to make 1 bar. This ore can be spawned in quantities of 13 in a diamond shape when a Corruptor and a Hallowor collide or in quantities of 29 when a Guardian Corruptor and an Aegis Hallowor collide. It is also dropped by Oblivion, in quantities of 150-250.
The appearance of this bar in the source mod: