You don't remember the reason because no reason has ever been provided. The Raven Queen's goals and motivations are left intentionally vague to give DMs a bit more wiggle room with her. Also, hexblades aren't the Raven Queen. Rather, they are sentient weapons that serve the Raven Queen.

Either way, I think I can come up with a few ideas for the kind of adventure you seek. Aside from being a warlock, what else can you tell me about your character?