What would happen if more than one creature dies, for example, rain of ? Does it clone the least powerful?
Actually, there is a slight delay in hits, if you've ever noticed with multi-target spells.
Since they actually individually and automatically target every enemy creature, it would probably copy the first one that died.
I've noticed that now, under a rain of fire, all creatures die at the same time (I've seen it with vultures, they don't grow). So I gess that if all creatures die at the same time, Tomb/Grave will copy one of them randomly.