Daemon Bones

Demon Bones (DB) are a reagent that is in use on Pagan. As the name already says, these are the bones of daemons, which makes aquisition of this reagent downright dangerous.

Pagan
The school of Sorcery found out, that even after death, the bones of a daemon retain the powers found in them during life. In a perverse twist this resulted in them being essential in spell for elements of summoning and binding, primarily of other daemons. Since a demon has to be slain to gain these bones -a task very lethal as the daemon won't part with them peacefully- it is the most difficult reagent to aquire for a Sorcerer.

Ultima Online
When the Stygian Abyss expansion opened, it was found that the gargoyle Mysticism school of magic makes use of demon bones as a reagent.

Trivia

 * Adding demon bones to any spell where they are not required can have bad consequences. In the worst case, a daemon appears to kill the sorcerer for using the bones of its kin.