Demeter was an Immortal.
After they were no longer worshipped as gods, Demeter and some of the rest of her family decided to live like mortals on Earth. However, when Hades tried to take over the world in the 20th century, she tried to contact Zeus to stop him. Though it was not immediate, Zeus finally returned and banished Hades to the underworld forever. Demeter and the other Immortals still on Earth decided to leave with Zeus when he went back to his alternate dimension. (PROSE: Deadly Reunion)