Blackened Heart

Corruption waits for the perfect moment to strike.
You reach out towards a living creature that you can see. The target must make a Constitution saving throw. If it fails, its body is filled with vile poison, and it takes 10d6 poison damage at the start of its next turn. If the target is reduced to 0 hit points before the start of its next turn, the poison explodes outward from it in a shower of disgusting bile. Creatures within a 10-foot radius of the target must make a Dexterity saving throw. If they fail, they take 10d6 poison damage, or half as much on a successful save.

At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the damage of each phase of the spell increases by 2d6 per spell slot level above 3rd.