The Necromancy rune is a rune "created" by Clary Fairchild which gave her the ability to bring back the dead without the aid of a warlock. Her "creation" of this rune was made possible by her the pure angel blood in her system.
When Clary applied the rune on the skin of Valentine Morgenstern's corpse, she was met by some physical and magical resistance. She persisted and was able to finish the rune through sheer will, after which the lights around them flickered and sparked as some sort of reaction. This brought Valentine back to life, and though he was momentarily disoriented and surprised at being ripped back to life, he recovered fully after a short while. Resurrection through this rune also made him impervious to conventional deathblows, but Clary's deactivation of the rune ultimately killed Valentine again.