[johndallman]

Twisted Mana Generator [-15] is the disadvantageous form of a trait which can also be advantageous. Your aura is strange, and magic has peculiar and unpleasant effects on you, and when cast by you. This disadvantage appeared in Thaumatology for GURPS 4e, and is not mentioned in any other GURPS supplements that I have.

Your aura is twisted, and those who can perceive it usually react to you at -3. Any spellcasting you do is at -2, but others casing spells on you suffer no penalty. The effects on spells you cast, or that are cast on you, are unpredictable, but always unpleasant. Failed castings always count as critical failures, and the GM should improvise painful and unpleasant results, but stop short of killing you. Successful casting is possible, but energy cost is always increased by 1 point, and the effects are twisted to inconvenience you. Healing is painful and leaves terrible scars; attack spells may backlash for a point or two of damage; created matter will be foul and repulsive. On a critical success, the spell may actually work, but things will always be odd enough to create fear that something has gone wrong.

This disadvantage may not be Switchable. It seems mostly useful for creating NPCs with vast magical talent that they don’t dare use. That’s a legitimate feature of a magical setting, but I’d be surprised if it’s ever been used on a player character.