I’m working out this Theory and could use some help.
The lord of ravens will turn the key.
We have a feeling this could be Odyn. He is known as the hand of valor and Eyir has mentioned to Vol’jin that this being is responsible for making Vol’jin into the creature he is. We know Odyn uses Light and Vol’jin appears to be a creature of the Light but we can’t be sure of that yet.
We also know Odyn made a deal with a being in the Shadowlands, in order to see what is going on there. That being seems to be the boss of Bwomsamdi. Let’s call this being Mueh’zala for lack of a better name.
I know that Mueh’zala has a destiny with Aran being the one to kill him with the crystal sword but a I’m going to say that a being of death manifest in our world cannot die and will only return into the Shadowlands. I feel like part of this is precisely in order to fulfill this destiny because as far as we know, Mueh’zala can’t come into our reality. …until Odyn turns the key and opens the door for it.
I think Mueh’zala is also the being that whispered into Vol’jin when he was so close to death. Why would it want to pick Sylvanas? It had to know that she would burn the tree and kill all those Night Elves, so I imagine, they had something to do with the plan.
I’m also thinking about the Ragnorok story where Surtr plunges the sword into Asgarde and then Death inherit the Earth.
So, I’m thinking Mueh’zala and Odyn are working together because there may be some sort of infection in the Shadowlands. They both can see it (literally). Mueh’zala whispers to Vol’jin and Odyn converts him into this Valkyr-like being. We also know Sylvanas obtains a dagger and I think she is going to help bring Mueh’zala from the Shadowlands to Azeroth. With the help of Mueh’zala, Sylvanas will have the powers of greater than the Lich King. Odyn is on board with this plan, keep in mind, he doesn’t really seem like he is a champion of the mortals on the planet. He seems to only be interested in the best warriors (kind of like Arthas). Odyn’s goals has always been to defeat the Old Gods, and this is the only way to do it. Death is immune to the Void and are the perfect weapon against the Old God (who we know is waking up).