On a successful saving throw the frozen soul regains the sacrificed hit points.
On a fail, the target is incapacitated and loses control of its body and gains the sacrificed hit points as temporary hit points. The frozen soul now controls the body and deprives the target of awareness.
If the targeted creature is dead it does not make a saving throw. The target gains immunity to being charmed and frightened, and retains all class features and proficiencies.
The possession lasts until the body drops to 0 temporary hit points, or the frozen soul ends it as a bonus action, healing any remaining temporary hit points. The target is immune to this frozen soul's Split Soul for 24 hours after succeeding on the saving throw or after the possession ends. The frozen soul can only have a maximum of two creatures affected by Split Soul at any one time.
Frozen Soul Lore
Beyond the whistling of the arctic winds, a horrific wailing can be heard as a large shadowy figure approaches. Appearing as bones and hanging flesh, the creature has fallen from the warmth of life. It shrieks a startling noise as the swirling blue energy behind the exposed rib cage deforms into humanoid faces. It’s energy lashes outwards towards the warmth it craves.
Remains of the Frozen. The frozen soul is created when a wanderer dies in the arctic by freezing to death. The persons soul wants for the warmth of life, and when many of these souls gather together they unite into a frozen soul. The only way to put the souls to rest is to kill their undead form.
Well of Souls. The frozen soul is not the abomination it has reanimated, but the glowing energy inside. The soul is able to split into multiple fragments, and these fragments are able to possess new corpses, or even possess the living.
Puppet Masters. The frozen soul enjoys taking control of the living and force them to fight each other to the death. However if the living prove to be too resilient the frozen soul will raise the corpses of creatures it can reach. The frozen soul is very likely to possess other beings to fight its battles.
Habitat. The home of the frozen soul can be identified by the littering of corpses. The soul is likely to kill people or beasts in the location that the souls it is made of died. This area is avoided by all who know of it, and often people leave warning signs to signal from wanderers to avoid the area.
Undead Nature. The frozen soul doesn’t require air, food, drink or sleep.