Decay, Defile, and Rot

You imbue the target creature with immense necrotic energy, causing its body to begin unnaturally failing and magically rotting even while alive. What a horrifying fate...
The target makes a Wisdom saving throw against your spell save DC, on a failure the spell takes effect.
The creatures current health point maximum cannot be increased by any means while this spell persists, except by leveling up.
Any time its health it would be restored it has to make on a Constitution saving throw against your spell save DC, on success the healing is reduced by half, otherwise it fails completely.

Every short and long rest, the creature rolls a 1d6 and adds your spellcasting ability modifier. It's health point maximum is reduced by this amount while the spell persists. 
Every long rest, the target has to succeed on a Constitution saving throw against your spell save DC or gain no benefits from it and have one of its highest ability scores reduced by 2 while the spell persists. (determined randomly)
After the targets health maximum is reduced by:
- one quarter, all of its its ability checks have disadvantage.
- half, all of its attack rolls have disadvantage.
- three quarters, all of its saving throws have disadvantage.

The creature dies if its health maximum or one of its ability scores reaches 0. It can only be brought back to life with a spell of equal or higher level than the spell slot used for this spell within 7 days of dying.
After this period of time, it rises as a ghoul, its bite attack forces the target to make a Wisdom saving throw against your spell save DC or suffer the effects of this spell. If another creature spends 10 minutes within 30 feet of the ghoul, it must make a Wisdom saving throw against your spell save DC, on success it is immune to this effect for 24 hours, otherwise it is also affected by this spell. In any case, it is unaware of this aura.

This spell can only be removed by a Greater Restoration or Remove Curse cast at equal or higher level that the spell slot used for this spell, or Wish,