(Note, this can't simply be something so strong that it kills anyone
outright if reflected, that's a Dangerous Forbidden Technique
This refers to something that because of the character's unique physiology, would be somewhat powerful on your opponents, but absolutely devastating to you. For example, if a ice-based creature started casting fire magic, an aquatic creature cast lightning, an angel cast dark magic, or a warrior in heavy plate armor cast lightning.
All that it would take for an instant death is a simple spell reflection. So why would they do this?
Alternately, if it's literature or a video game, they may either doing this so the priest isn't so useless
, or to give a viable reason for the hero with no attack spells winning against someone who's powerful in a wide range of magic.