I couldn't miss the chance to practice my drawing!
Onimaru is a level 60 katana that provides +34 Bravery, +58 Attack, -1 Defense, and EX Intake Range +1m. However, the weapon can only be obtained, and used, within the Labyrinth. It can be obtained from the shop by trading 1 gil, Sorrow of Conflict, and Giant's Dream x2.
Oni is the kun'yomi rendering of the Chinese interpretation of an ogre called the "guǐ" (gu-ay). They are a kind of yōkai from Japanese folklore, variously translated as demons, devils, ogres or trolls. In a Chinese context it may refer to a ghost as well.
Maru means "circle" in Japanese.