Soulmates found one story at a time.

Mages are men who can use magic, but don’t have any of their own. They steal it from witches and magical creatures to power glyphs etched into their souls. 

If they don’t drain power from a witch, the glyphs on them will feed on their physical bodies, which drains them of color and turns their bodies skeletal. As a result, mages must find new vessels frequently. When the magic trades bodies, the consciousnesses of all the prior  mages transfer with the magic.

Mage magic can’t be shared and must be taken.