Calcified Spikes

Choose a creature you can see within range. Their bones calcify into spikes, shredding the target from the inside. The target must make a constitution saving throw. If they fail, they take 2d8 slashing damage or half as much on a successful save. On each subsequent turn, you can use your bonus action to make them repeat the saving throw, taking 2d8 slashing damage again on a failed save or half that on a successful one.

If you cast the spell targeting undead with a bone structure, they instead grow spines that damage anyone that goes near them. When a creature makes a melee attack against the target, they take 2d8 slashing damage. When the target makes a makes a melee attack, that attack deals a extra 2d8 slashing damage.