I wish they would put something out that combines all the cosmology info in one spot. While doing research on Vecna the other day I had five books spread out in front of me and i was bouncing from one to another…
The most info on The Raven Quenn can be found in The Manual of the Planes, particularly the part about the Shadowfell. However, there is a nice little nugget in the part about dead realms that describes how she was once mortal and had been selected by the former god of death Nerull (!) to be his bride but she defeated Nerull and usurped his position as the godess of death.
Hope that helps.
Doug
