My idea is just adding a new Aura when in Reaper form;
The Reaper’s Presence:
+10% damage when attacking an enemy under 50% health
+20% damage when attacking an enemy under 10% health
Max effect of 5 Allies
500 Radius
It go’s with the theme of the reaper being a sort of “grim reaper,” while still requiring him to be close to get the extra damage for himself and each ally.