Gorgon

The Gorgons were a race of humanoids that entered human Greek mythology, as their name suggests. One gaze from the Gorgon's eye and the person will be turned to stone. (SJA: Eye of the Gorgon)

Biology
The Gorgons could possess people, but in reality they look like ethereal serpents. Once a Gorgon finds a host, it can xpand the host's lifespan.

History
Three Gorgons came to Earth three thousand years ago via a strange talisman. One was killed by a greek hero, one slain by a professor and his wife in 1959, but there was one surviving one. This Gorgon disguised as an abbess and formed a sisterhood of nuns to protect it. Since then, it has sent the nuns to find the talisman so it can escape Earth. (SJA: Eye of the Gorgon)