Transmogrifying Curse

You inflict a horrible abyssal curse. This curse only affects Beasts and Humanoids. The target must succeed on a Constitution saving throw or take 4d12 Necrotic damage and have the Poisoned condition. Also, the target's maximum Hit Points are reduced by half the Necrotic damage taken.

The target must repeat the saving throw at the end of each of its turns until it gets six successes or failures. If the target succeeds on six of these saves, the spell ends on the target. If the target fails six of the saves, they shape-shift into a Fiend (Demon) of Challenge Rating 6 or lower under your control.