The Raven Queen is the goddess of death. She has appeared in front of the heroes as a kind of living nest, with darkness where her face should be. She gave Paultin the severed hand of Ch'gakare. Her elves made the ring of winter. 
The waffle crew learned that long ago, the Raven Queen and a few elves tried to cast a big spell, but it did not go off as intended. In despair, she sunk into the Shadowfell. She put a lot into the spell and had been greatly weakened.
She became a deity bound to the Shadowfell, an emblem of death who uses ravens to make sure souls get where they need to go. Sometimes she locks away special souls to keep - people of destiny. Good and evil people. She helps them achieve their full destiny. 
When the group was on the beach in the Shadowfell, the Raven Queen came to collect the skull chalice and the hand of Ch'gakare. In exchange, she gave Simon the ability to speak. The Raven Queen spoke Celestial, which Evelyn was able to translate through.