Vampirfürst (Civ4 FFH)
This unit requires a higher level unit than the Immortal that it replaces, but it is far more powerful, largely due to the base promotions (Immortal is the only one possessed by the Immortal unit).
All vampires have unnaturally long life-spans, but the vampire lords have learned to cheat death itself. Whether through massive feasting or some more arcane ritual, the body of a vampire lord is recreated upon destruction. Some spend their stolen years further honing their martial prowess, but many, having experienced everything else in life, study forbidden knowledge, learning to cast advanced death and body spells without mana. A few take the final step towards immortality, and become a Lich.