Thor 2 AU prompt, because I wanted a very different Jane here.
Even though Malekith has been defeated by Bor and sent into a death sleep, there was another whom Asgard forgot.
An abandoned, mortal child (Jane) whom Malekith took and raised for his own (after his own family was killed by the Asgardian army), has Aether running in her blood. (As the Aether protects itself, it inadvertently protects her, prolonging her lifespan) Jane was always meant to aid him in his plan to return the universe to darkness, but she fell into the same death sleep by Malekith's side after the defeat of the Dark Elves until the next convergence.
She awakens at a crucial time several thousand years later, stirred by the Aether that's responding to the coming convergence of the realms. Loyal to a fault, Jane recognises that the Dark Elves can succeed yet another time - and that her participation is crucial.
Asgard, under Odin and Thor, is unprepared for this awakening darkness as they deal with Loki's trail of mischief and destruction. Jane sends herself into Asgardian prison, biding her time and there, she meets the infamous trickster who's also imprisoned. In him, she thinks Malekith might have a worthy ally.
But Loki is slippery and unpredictable and near impossible to sway. He knows that the Aether is Malekith's driving force and capturing it could mean the restoration of his standing in Asgard.
In this tussle of wills that begins in an Asgardian prison, who's going to win?