Astral snap

When casting a spell you partially focus your consciousness and the spell into the astral realm, snapping it back at the moment of impact.  When you roll damage for this spell instead choose the maximum possible.  The strain causes a con (14 + the spells level) save.  On fail you take 2d10 damage and gain a level of exhaustion.  On pass you take half as much damage and no other effects.  Once per long rest.