The caster cast a point-target (or AoE) ability, and release a Death Coil towards the point/AoE. When multiple enemies are hit by Death Coil, the damage is distributed between them all, and the caster is healed for the damage done. Only a maximum of 3 units can be damaged by Death Coil at once. However, if no unit gets hit by the Death Coil, the caster will lose mana, and he will not be healed.
The player has no control over which unit is hit by Death Coil when cast on a group of enemies.
P.S. Also, can you create a variant of the spell where 4 units are hit, and more damage is dealt?
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Interesting idea. Good luck on your project