Warning: The following contains spoilers for Valkyrie: Jane Foster #10 by Jason Aaron, Torunn Gronbekk, Ramon Rosanas, Jesus Aburtov, and Joe Sabino, available now.
During her time as Thor, Jane Foster wielded the Asgardian Thunder god's iconic hammer, Mjolnir. Even though it's one of the mightiest weapons in the Marvel Universe, Mjolnir still isn't the most powerful weapon Jane Foster has wielded. As Valkyrie #10 makes abundantly clear, her All-Weapon can do things that Mjonir never could.
Jane Foster may be relatively new to her mantle as Valkyrie, yet she's already taken on some of the most threatening antagonists Marvel can throw at her. Most recently, she's faced the Røkkva, an ancient entity that's so powerful that it was strong enough to take over the likes of Thor and Tyr.
The Røkkva is the antithesis to life itself, and it had been buried, alongside the Asgardian King who tried to wield the power for himself, in the Barren Mountains of Jotunheim. That is, it was until Tyr Odinson, the erstwhile older brother of Thor and Loki, hatched a scheme. Over time, Tyr became bitter and enraged with everything Thor had, so he devised a plan that would give him all the power he's ever desired while striking at everything his younger brother cherished. He brought the fallen king and Røkkva to Midgard, knowing full well the havoc it would create upon the earth.
Once unleashed, very little that could stand in the Røkkva's way, not even the increasingly powerful Thor. The ancient evil possessed Thor, alongside every other living creature it came in contact with, turning them into dark puppets in a very dangerous game. Suddenly, Tyr started looking a lot less like a controller and more like another puppet the Røkkva has taken charge of.
All of that sets the scene for Jane Foster's plight. Her All-Weapon, Undrajarn was lost to her when she first battled Thor in his newly possessed state. Worried about her friends, family, and her status as a Valkyrie, Jane still attacked the Røkkva possessed Tyr.
In hindsight, it probably was not terribly smart of Tyr to try and wield Jane's own weapon against her. The All-Weapon may have been infused with primordial darkness, but it still knew her, and she knew it. Once the weapon came in contact with her flesh, the rest of the battle became set in stone.
Jane and Undrajarn are one. She could feel the weapon's struggle in her bones, even before she was brought back into contact with it. As weapon and woman, they were enough to force the Røkkva back into a smaller, sleeping state and place it back where it belonged.
This whole sequence of events proves that Undrajarn is no ordinary weapon. It was already clear why it is called the All-Weapon, thanks to its ability to shape itself into literally any weapon that Valkyrie needs. Now, we know that it can also form into weapons and items long lost, such as the White Wood of Feierheim, the only weapon capable of destroying a fallen king's body, and the key to so much more.
It was the only weapon strong enough to fully unleash Røkkva, and likewise, it was the only one capable of trapping it away once again. This is a task that no other weapon could have managed, not even Mjolnir.
The All-Weapon is arguably the most powerful new weapon to debut in recent memory, and it's just as versatile as it is dangerous. The ability to shift into any needed weapon makes it ideal for any rising hero, and the bond it carries with Jane makes it uniquely equipped to share that power on a deeper level. Accordingly, there can be no doubt that Undrajarn is the most powerful weapon Jane Foster has ever wielded.