If {Prey Upon} is used against a creature with {Spirit Mantle} what happens? Just to be sure. It's an illegal target, right?
It's not an illegal target, but protection from creatures will prevent the damage from being done to the creature with {Spirit Mantle} on it. It will essentially end up as a one way fight, since only one creature (the one with the mantle) can deal damage to the other.
However, if the fight was caused by {Nightfall Predator} or {Ulvenwald Tracker} then it would indeed be an illegal target (due to the protection from creatures) meaning it could not happen.