The Spell of Life is what gives immortality to the first Dynasties, that were created before the Dja-akh struck the Underworld. It was designed by Isis and, while it was able to substain a body for millennia, it was flawed in that the recipient became a half-dead creature, resembling a walking corpse.
The true magic of the spell of life is powerful stuff from the time when the Mythic Age was undaunted and the worlds undivided. Isis was acutely aware of the natural cycle as her magic style was far closer to what modern Verbena practice than any other magical Tradition. She knew that all things must be born of the dual powers of the male and female and that all things must eventually pass onto the next stage of life through the doorway of death. For this reason, the feral allies of the Shemsu-Heru regarded their immortality not in the same way as they regarded those of the Leeches, for the Mummies followed the natural cycle by dying and rebirthing. (It is worth noting that such shapeshifters are in the minority, and to most Garou and members of all of the other Changing Breeds, the mummies are blasphemers against Gaia's sacred law, that the dead must remain dead. The Spell of Life, to those who have heard of it, is seen as a means of corruption by which the Wyrm of Balance was fooled into allowing this crime against Gaia.) Thus all Immortals created with the Spell of Life must die before they become Undying, and not until they have died can they become Undying.
Each group or camp of Mages that practiced it have added their own elements, beliefs and foci, this created the various dynasties, for each Dynasty had used a different variant to become immortal.
After Osiris awoke, he granted a new Spell of Life which did not contain the flaws of the previous one and filled the recipients with lifeforce and vitality, thus creating the Amenti. Those who denied this spell were declared Shuankhsen and are hunted by the Hand of Thoth.