Dr. Kobold's Vexing Scalpel

Straight from the pint sized physician's machinations you summon a thin onyx black blade with a thick green fluorescent ooze dripping off it. On a hit the target takes 4d4 necrotic damage and 2d4 slashing damage. The wound left behind will rapidly fester forming large blisters and boils that after the targets next turn will erupt dealing 3d12 acid damage to all creatures in a 10 ft radius. The target can prevent this by using its action to drink a healing potion or if a healing spell is cast on them the boils fade harmlessly.

At higher levels the damage dice increase by one damage dice until it is cast at 5th level where the d4 damage dice becomes a d6. If cast at 9th level the spell becomes permanent for 1d4-1 days (Minimum of 1 day)