This seems only doable in JASS, because you have to get the event damage. For the caster not to take damage but still get the event damage, you temporarily set his life very high, test for the event damage, do what the Aphotic Shield does, then set his life back to normal. No one would notice because it is only a split second trigger.
I do not know how to make this spell MUI either because of using a global variable.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.