Keres is the female death-spirit that lurks around Atlantis. She will appear to you when you are ready to build the Spectral Ruins. She explains the dark details of building the Spectral Ruins and resurrecting the dead.
She is always at war with Cassandra. Keres often announces events occuring in Atlantis.
She is also the person that casts curses upon your enemies. You can buy curses from the Shop for 9 rubies each or they are frequently won from Fortuna's Chance.