Life Force Transfer

Upon completion of this long and highly-versatile incantation, the caster permanently transfers a creature's life force (even his or her own) into a specially fabricated item, a magical receptacle, or the body of another individual. Once the mental transfer has been completed, the recipient's body falls into a cataleptic state, and remains in a death-like trance or coma for 2-7 days. Unless the subject's life force is magically reunited with the body during that time period, his or her body dies, and the life force remains permanently trapped within the physical object or new body.
While in the physical receptacle, the recipient may perform any action permitted by the new form. In an item such as a ring or a sword, the subject may communicate with the user when the receptacle is held or worn properly. The extent of communication depends on the subject's intelligence:

Int	Communication
3-11	None
12-13	Semi-Empathy
16	Empathy
17+	Telepathy

Individuals of low or average intelligence cannot communicate, and those of above-average intelligence can communicate using a primitive form of empathy (usually limited to a throb or tingle of varying intensity). At the discretion of the DM, the life force might animate a specially prepared statue or golem, perhaps even enabling the recipient to employ any memorized spells.
Life force transfer is sometimes the final step in the manufacture of a powerful magical item or minor artifact (usually a sword). In this process, a victim's life force is transferred into a magical item (unless he or she saves vs. death magic at -4), which is finally sealed with a Permanency spell.
In such an enchantment, the recipient's body is frequently destroyed afterward by the caster, forever trapping the victim's life force in the item (unless the permanency is first reversed and a new body prepared using a Clone, Wish, or similar means). If the item is destroyed while the recipient's life force is in it, the creature may regain its body by making a system shock roll if it lies within 1 mile per level of the original caster; otherwise, the subject's life force dissipates, and the creature dies.
Sometimes, a wizard may employ this spell as a final measure to protect his or her own life. In this version, the entire incantation and receptacle are prepared and cast beforehand, but only the final syllable of the spell is left unfinished. The wizard may pronounce this short (single segment) syllable at any later time, casting his or her life force into the receptacle — providing that it lies within range. Obviously, this strategy only works if the receptacle is kept close to the wizard at all times. Furthermore, the wizard usually must leave behind clues or encrypted instructions so that his or her allies may later reverse the spell and restore the wizard to his or her normal state, whatever that may be. While the spell is in its unfinished state, the wizard cannot learn a new 8th-level spell to replace it. Aside from this limitation, however, the caster remains unaffected by the pending spell.
Less scrupulous wizards may use this rite to permanently transfer the recipient's life force (often their own) into the body of a younger individual, thereby extending their own existence. The victim is entitled to a saving throw vs. death magic with a -4 penalty to resist the effects. If failed, the subject's life force departs (resulting in the death of the victim), and the recipient's life force takes permanent possession of the victim's body. If the saving throw is successful, the spell fails without any ill effect to either the victim or recipient (though it may be recast at a later date).
Thus, an ancient necromancer may transfer his or her life force into the body of a vigorous, younger person, thereby gaining all of the victim's physical attributes, manual skills, hit points, and extended life span.

Material Components

The exact nature of any physical receptacle should be decided by the DM, but it must be of quality suitable for enchantment. The garnet (but not the scroll) is consumed in the casting. This spell cannot be dispelled normally (even in its "pending" state). If the receptacle was not sealed with a permanency, then the subject's life force may be driven out with Spirit Release or Dispel Evil (in the event of a pemanent magical receptacle, the item's permanency must be first overcome with dispel magic).
In rare cases, to be adjudicated by the DM, the life force of the victim does not depart from the body, but coexists in his or her body with that of the recipient or "intruder." Such a life force may be able to take control of the survivor — use the control rules for the Magic Jar spell to determine this. As before, the "intruder" life force can only be ejected by casting spirit release or dispel evil (or a full Wish). Otherwise, at the discretion of the DM, the life force of the "intruder" merges with that of the victim, diluting the recipient's personality, alignment, and possibly sanity, as well.

Reverse: Revoke Life Force Transfer

The reverse of this spell, revoke life force transfer, requires the original scroll (the one used in the life force transfer spell to be revoked), the construct, and the body of the recipient. Providing that the spell is cast before the body physically dies (within 2 — 7 days of the original separation), the reversal process requires a system shock roll in order to be successful. If failed (or if the body has already perished), the newly released life force dissipates, and the subject dies.

[Complete Book of Necromancers]

Reversible

School: Necromancy
Level: 8
Rarity: Necromancers: Uncommon
Others: Very Rare

Range: 10 feet
Components: V, S, M
Duration: Permanent
Casting Time: Special (8 turns)
Area of Effect: 1 creature
Saving Throw: Negates

Material Components:
Life Force Transfer: an expertly-crafted item or any living human, demihuman, or humanoid (preferably young and healthy) to house the subject's life force, a scroll inscribed with special rare inks and a massive, blood red gamet (worth 5,000 gp or more)
Reverse: The original scroll used to cast the Life Force Transfer