The Raven Queen

"You stand before the keeper of memories, collector of precious mementos; and gatekeeper of mortal souls. The Queen of Death welcomes you to the city of silver and bone." -The Raven Queen's Speaker

First encountered directly by the party in Arc 2, Session 20. The party found themselves unexpectedly in the Shadowfel after exploring the ruins of Shae Tirias Tolai, deep within the underdark below Xen'Drik. The ruins seemed to be a variation of the true city within the Shadowfel, and a fountain of bone and silver; adorned with skulls that appeared to represent the party. As the party's minds were assailed in blinding pain, they hear the sound of a giggle, and shattering glass.
Her followers and symbology are, understandably, all raven themed. Her followers that the party encountered in Shae Tirias Tolai also wore raven masks and long dark robes, obscuring their features completely.

Descriptions

If the Raven Queen has a true form, it's uncertain if any of them saw it. The only consistent feature was the hair, long and raven black. She first appeared as a child, a pale elf girl with pitch black eyes. Later, at the throne, she appeared as a glowing gray orb; sprouting her hair as though tentacles or snakes. Her voice was a cacophony of female voices all speaking at once; with one slightly clearer and understandable. Brian saw a third form, when he agreed to give her a memory, of an adult elf woman, her face obscured by long black hair.

Deals and Doors

The Raven Queen seemed to put the party through some sort of trial, or game, as they entered the Shadowfel. Choices were presented to each of them as three images, or doors. It didn't seem to matter what door each selected however, as the party found themselves regrouped within the Shadowfel afterwards.
Later, in the throne room, Brian, Fork, and Kerani were given the opportunity to make a request. Depending on the request, the Raven Queen would state a price, and then the requester was allowed to modify the offer once. She, and her speaker, were insistent on these rules, and even interrupted Tilly's attempt to magically speak out of turn.
As payment, she requested a variety of things. From Brian, a memory; which she did indeed demonstrate having the power to take when he agreed to the terms. From Fork, a promise of aid against Bastimaar. From Kerani, she required the object she seeks, the Teeth of Three, be ultimately brought to the Raven Queen.