Relic

Relics are small pieces of powerful wizards, used by arcane spellcasters to store magical energy from votive crypts. These could be as simple as a finger bone or tooth of a court wizard, or could be as powerful as the mummified head of a great necromancer. They are often set in some sort of jewelery or accessory- it is unusual even for the eccentrics typically drawn to arcane spellcasting to just tote a preserved tongue around in their pocket.

The bodies of contemporary spellcasters are not suitable for relics. It seems that only remains found in ancient ruins have the power to function as such. This makes raiding ruins potentially extremely lucrative, as powerful relics are rare and valuable.