Who is the goddess of the dead?

There is no goddess of death. There are several female spirits associated with war and suffering, and some goddesses associated with the underworld, like Persephone, queen of the Underworld, and Hecate, goddess of magic, but no "goddess of death." Thanatos is the personification of death, what we today think of as the Grim Reaper, and Hades was lord of the underworld and therefore the dead.