This spell was inspired by Autoparthenogenesis, a spell from Space-Age Sorcery:
There's a lot of neat stuff (and a lot of horrible squicky stuff) there- you should take a look at it, if, like me, you are into adding doses of Lovecraft and/or Science Fantasy into your game.
Arcane Ritual 7
The grim rebirth spell is an arcane answer to resurrection and regeneration in many ways. Once the magic user casts the grim rebirth ritual, a part of their spirit is locked in time, exactly as it is at the time of casting. After this, at any point in the future, the wizard may "complete" the Rebirth. This causes their flesh and bones to burst outward and tear away as the part of them that was locked away creates a new body instantly within them.
This new incarnation is in every way identical to the caster as he was at the time of the ritual's completion, save for some memory of any new knowledge gained. Experience points are lost, and must be regained, as must any new spells added to repertoire, magic item research, etc. Ephemeral things, such as what year it is, and any trivial knowledge learned during adventuring are retained. Only one casting of grim rebirth may be active at any time. If the ritual is cast again, the previously cast grim rebirth activates, possibly regressing the caster against his will.
Example: Arcagen the Artificer casts grim rebirth upon his earliest opportunity, locking in an 11th level version of himself. He then proceeds to make a name for himself as a creator of a new type of silver golem, which he uses to terrify the countryside.
Six years later as a 13th level mage, surrounded by his foes, having lost an eye and beaten bloody, he speaks the key word to complete the ritual. His enemies are shocked and disgusted as a younger version of Arcagen steps out of the ragged scraps of flesh that were left behind- this one with full hit points, and a full mind of uncast spells. This Arcagen is 11th level, exactly as he was at the time of casting. He remembers, dimly, that he has created his silver golems, but frustratingly will need to use his own research notes to re-learn the process if he is to make any more.