Raven Queen is not a Forgotten Realms Deity. She is just some being from the Shadowfell.
I think you completely misunderstand...the "Forgotten Realms" is just a different iteration of the D&D Material Plane...that has nothing to do with all the other actual planes of existence in D&D...Shadowfell is a different plane of existence that exists alongside any and all D&D interpretations of the Material Plane. Shadowfell, the same as the elemental planes and all the rest are
all there.
