The Rite of the Revenant was an ancient ritual which could bring a dead person back to life usually as a lich (a spirit who possesses a host body which often decays over time) or as a revenant (the spirit is brought back to "unlife" in its own body or a close replica of it like a clone). However, the spell required powerful and talented people to cast it, and it also required a tremendous sacrifice from the casters because of its unholy nature.