I was the one who suggested a Restoration type spell, that would be similar in reverse to Draw Forth the Soul, presented on page 204 of my 'old' Conan RPG tome (I will order an Atlantean Ed through Sons of Cimmeria).
The base spell mechanics for the spell would be similar, except a non- corrupt Sorceror (Or Mitran Priest or similar) would have different Pre-Requisites. The spell is cast at night. The spell would count as a major spell, only castable once on the same target, and subject to the Mighty Spells and Runaway Magic risks.
The target must be recieved as a Left for Dead, or recently deceased (-10). Special preparatory salves and runes painted onto the body (while chanting the ritual of restoration. Hey maybe a new Sorcery feat?), preserve the body and reduce any hit point loss to only -1HPt per hour instead of every round - thus enabling the ritual to proceed. Once the Ritual starts there's no further hit point loss unless it fails. The target's Con is assumed to be down to 1.
The Power costs could be 20 to establish, followed by as many points as the difference between the original Con of the target and 1, per hour. The caster makes a magic attack roll against the Spirits of the Afterworld, that come to take the Soul of the target. Each successfull roll v's the Will (Plus modifiers) of the Spirits, cast each hour, restores an initial one point of Constitution to the target. The next successful magic roll restores two points, reducing the power point cost - and so on. Each failed roll vs the Spirits makes the cost one point higher. Success is achieved when the target has all of their Con restored to it's normal total, wherein they are taken to 1 hit point and must recover according to the healing rules on pages 172/173.
This is rather like a drawn out War of Souls. The spell must be cast within a certian time limit of the target's imminent 'final death'. Power rituals may keep the caster 'up; in power while the ritual is in progress (Table 8-1, page 190) - like with a circle of supporting celebrants (Maybe in a sacred grotto or temple).
Of course - if the dying/ dead target has their Soul already promised to a Demon, then the DC is set as the demon's will, plus it's corruption, and any other modifiers. The demon may decide to 'get even' with the interfering interloper.
This is the idea anyway ... anyone may make some modifications or changes. I think that it's sufficiently difficult to preclude it from being vetry commonly used. It's definately interfering in Fate, so inclusion in the Immortality spell class perhaps?
Pre-requisites could be: MAB of +7 or higher, Healing, Warding, Greater Warding. Priest or tribal equivalent)