Dethril

Dethril, the Metal of Darkness. It can grant the power of the demon...in exchange for one's soul. Dethril (デスリル Desuriru), known as the Metal of Darkness (闇の金属 Yami no Kinzoku), is a rare type of metal. It is known for enhancing the user's physical capabilities, granting phenomenal strength, vitality, and agility. However, the user's lifespan is gradually shortened, eventually ending their life in a painful process.

It is commonly excavated near Falgabard, a town (in)famous for producing powerful Dark Knights. That said, Dethril is a common material for creating Dark Knight equipment.

Trivia

 * Dethril is a portmanteau of death and -ril, a fictional suffix meaning "glitter," from which mithril is derived from.