Cascading Decay

You infuse a creature within range with an echoing necrotic energy. The target must make a Constitution saving throw. On a failure, they take 5d12 necrotic damage, and must make another saving throw or take one less dice of damage, repeating until out of dice. On a success, they take half damage for the current number of dice, and the spell ends. 

At Higher Levels: The initial damage increases by 1d12 for each level above 4th.