Create Deathless

You call forth the spirit of dead Humanoid, and attempt to bring their body into their corpse, ultimately creating an intelligent undead with the memories they once had in life, and not bound to your service.

This spell must be performed under the cover of nightfall, and must target a humanoid corpse. The creature must be willing, having agreed to this circumstance in life, or by communicating to them from beyond the grave. This is reflected in the ability check you make at the end of casting the spell. You place the required Moonstone into their chest, where their heart once was, and inscribe a circle of Arcane sigils and runes around the corpse.

At the end of the casting time, make a spellcasting ability check, the DC of which is equal to 10 plus the CR of the creature when they were last alive. The spell automatically fails if the soul is unwilling. On a failure, you take 5d8 necrotic damage, gain two levels of exhaustion, and the spell fails. You can not attempt this spell on this corpse ever again. On a success, the spirit is returned to the corpse, regaining the same statistics they possessed before they died, save for a few key changes.

The creature's type is changed to undead. They are immune to poison damage, and the poisoned and exhausted conditions. They no longer need to breath, eat, drink, or sleep. They no longer age, but their corporeal body will continue to decay as any other corpse or undead would. By agreeing to such magics, they also forfeit their souls, and cannot be raised back to full life by anything short of a Wish spell, and if they are destroyed as a Deathless their soul will not depart to the afterlife.