Death Release

You pick a non-undead creature to focus destructive energy from beyond on it. All physical attacks against the creature have an advantage. If that creature dies before caster's next turn, at the moment of death, it rolls a Constitution save:

On success, the creature manages to withstand the energy and hold it for a bit. The creature turns Undead with 1d12 HP. Now it must succeed Wisdom save or become controlled by the caster. At the end of its next round, the creature explodes.

On Fail, it explodes right away. 

Explosion dealing radial 3d6 damage in 20ft. radius.

The spell gains additional d6 for each extra level when plugged into higher-level spell slot.