Death's Destruction

After a haunting training session, Damon was granted access to the following spell:

Once a creature has reached death's door (10% of hp) Damon can reach into the very soul of a creature, filling it with arcane energy, creating a large explosion of necrotic power. The creature targeted is instantly killed, and all creatures within 20ft sphere of the explosion make a Dexterity saving throw, taking 10d6 necrotic damage on a failed save, or half as much on a success.

The caster can also cast this spell at a lower level, costing 1/3 of their maximum hit points for each level under third. (Healed on a short rest taking 2d6 hours after casting)

The damage of this spell increases by 1d6 for each spell slot above level 3.