What if...
All (This is a lie) the Gods have been killed, and their blood has been drunk. The immortals reign with divine, godly powers, many as cruel tyrants, as the blood of the divine was not meant for mortal hands. Those power-hungry enough to wish for their own immortality must drink the blood of these new immortals. A little bit will grant magic- but all of it (enough you need to kill them) will turn you into an immortal yourself.