Raven Queen is not a Forgotten Realms Deity. She is just some being from the Shadowfell.
That’s a reason for her having Warlocks and not clerics. It’s not unknown that a far realm being having warlocks to further their influence beyond their realm. I can see (not really the Raven Queen), a deity from another plane sign a contract with a Warlock to set up a following and/or build a temple to start up a fledgling religious order.
But, since she isn't a fiend, a fey or a GOO she aint in. There was a Raven warlock in a WotC Unearthed Arcana, but it didn't make the cut. So there is just a vague reference in hexblade, witch has been subsumed into pact of the Blade. Raven Queen is cool and all, but I don't see a place for her in this story.
Tharizdun, mentioned as a potential GOO patron, is a Greyhawk deity, but has been sighted in the Forgotten Reals in the guise of The Elemental Evil Eye (Princes of the Apocalypse).
