Have a look at Ereshkigal from the old Babylonian mythos as the female ruler of the Underworld (Irkalla). You can find a lot of inspiration there and probably also some of the inspiration of the Raven Queen character. According to the old Babylonians the people that entered the realm of the dead (Irkalla) were having cloaks of black feathers, and was eating ashes from the ground like birds. Ereshkigal can also explain the Raven Queens' neutral alignment, as the belief in ancient Babylonia was that the rich or the poor, all got the same fate in the afterlife. So rather than looking forward to the afterlife, the message of the gods was "enjoy life while it lasts" (imagine the consequence of a society with such a belief ...). The whole story of Gilgamesh's failure to fetch his dead friend from the underworld, also serve the message that no living can avoid death, which of course can be a key driving force for the Raven Queen to actually claim the soul of Vecna that has avoided death!