Cellular Decay

By focusing your attention on a single creature, you summon forth the Weave inside it's very being, slowly breaking down its physical form. The target must make a Constitution Saving Throw. On a successful save, the target takes 2d4 Acid and 2d4 Necrotic damage, and the spell ends. On a failed save, the target takes 3d4 Acid Damage and 3d4 Necrotic damage, and until the spell ends, you can use your action on each of your turns to automatically deal 3d4 Acid and 3d4 Necrotic damage to the target. At the beginning of each of the target's turns, they must make another Constitution Saving Throw. On a success, the spell continues as normal. On a fail, the automatic damage increases by 1d4 for each damage type. If a creature is killed by this damage, it's form dissolves into sludge.

The spell ends if you use your action to do anything else, if the target is outside the spell’s range at the end of your turn, or if the target is not in your line of sight at the end of your turn. If the target is out of range or out of sight, you cannot deal the automatic damage to it.

(As an example, after 5 rounds of failed Con saving throws, the creature would automatically be taking 8d4 Acid and 8d4 Necrotic damage. On the last (10th) round, assuming it failed every single save, it would take 13d4 Acid and 13d4 Necrotic!)