Normally, the relationship ends in the mage becoming an abomination. Or using filthy blood magic to establish his goals. It is normally a futile effort, though a dangerous one.
[Hmph. Mages.]
Though I suppose not all worlds have abominations or blood magic.
[And Fenris could explain, but.]
Know that your sense of self and will are erased because of it. You become their puppet should they see need of it.