Limen Magus

Limen Magus (リメン マガス, rimen magasu, lit Instant Magician) also known as the Forbidden Dragon of Demonic Origin's Library is a Sacred Gear which contains the soul of the Evil Dragon Aži Dahāka which makes it an Longinus Candidate

Summary
Like most of the Evil Dragons of the Evil Dragon Set Aži Dahāka doesn't know how he got put into a Sacred Gear.