Obsidian Spear is an item in Wild Arms 4 and Wild Arms 5, one of the four items necessary to battle Ragu O Ragula.
Wild Arms 4[]
In Wild Arms 4, the spear can only be found and purchased in the Black Market in Guara Bobelo, in exchange for 40 of the party members' cumulative levels. It must be used at Etemenanki to break the seal along with the Crystal Tear, Holly Crown and Sacred Shroud.
Wild Arms 5[]
In Wild Arms 5, the spear must be brought to the deepest part of the Abyss and placed on a pedestal along with the Holly Crown, Sacred Shroud and Jasper Pillar.