Corpse Explosion

As you crush the skull of the small creature all corpses of size small or larger, within range start exploding. Any creature within 10 feet of the corpse must succeed on a DEX save or take 5d4 damage, half as much on a success.

This works on living zombies too, if the zombie succeed on a CON save they dont explode, otherwise they will explode with the rest.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 2d4 for each slot level above 2nd.