When you begin to die after failing a Death Test (this is after all attempts to use Inevitable Comeback have failed), make a Supernatural Creature Powers check in place of the Medicine check that would normally be made to attempt to save your life. If this check succeeds, you disappear, much as if you had been using the Dematerialization schtick, only to reappear in a later session.

Creatures with this power always have some unique nemesis, and if this method or item is used to kill them, Immortality will not work.

Immortality kicks in some time after the fight where you were killed. If somebody decides to give your corpse a parting shot, it dematerializes right after that attack.

Note that spirits and entities without this schtick is in trouble if they fail a Death Test and no mystic healing is around, as a normal doctor cannot help them. A doctor can save the life of a normal supernatural creature, one that is not a spirit or entity.