Fire Kingdom

(Inspired by RWBY: Ice Queendom, Silent Hill, and the Destiny franchise)

In this story, it's the reverse of Ice Queendom.

In this story, it's Jaune who's hit with the Nightmare Grimm... after Volume 9, in the deserts of Vacuo.

Team RWBY, particularly Ruby and Weiss, know of the hell that Jaune went through in the Ever After, while the rest of his team, particularly Nora and Ren, know of his struggles and pains that he's been going through since the Fall of Beacon, particularly the loss of Pyrrha and how he had to assist in Penny's passing to keep the Winter Maiden powers out of Cinder's hands.

So, using the knowledge and experience that they gained from their last venture into someone's nightmare world, Team RWBY, Nora, and Ren, enter into Jaune's Nightmare Realm, intent on freeing their friend from whatever fate may have befallen him...

Only to find themselves lost in an utterly alien world.

The Nightmare Grimm has had a massive well of Aura to feast upon, and has constructed a vast labyrinth within Jaune's mind to enthrall and entrap him within. Even worse, it has begun to create illusions based on his past traumas, both on Remnant and during his many years in the Ever After, feeding on his darkest thoughts and despair to torment and break him even further.

The end result? A massive world that has grown and evolved far beyond the scope of anything that our heroes have ever encountered before, ranging from a sprawling graveyard filled with statues dedicated to the dead and the dying to an expansive city made of bone that is constantly hounded by Grimm clawing at the gates, their only line of defence being an army of warriors known as the Pyrrhus.

Then, beyond that are layers upon layers of dark catacombs and black prisons under a constant night sky, like a sprawling dungeon constantly going down and down and down into the darkness until there is nothing but hollow words and dead dreams personified as constantly rotting corpses. Each layer is progressively more dangerous than the last, filled with traps and unnatural terrors.

(Essentially, this world is a mixture of the town of Silent Hill from the franchise of the same name, the Throne World and the Dreaming City from Destiny 2, and the various Black Fleet architecture from Destiny as well, particularly from the Sundered Doctrine dungeon and Vow of the Disciple raid.)

Team RWBYNR find themselves scattered across this wide world, either lost in the outskirts or held in the city or making their way through the labyrinths below, witnessing the world grow around them as they find the people in the city constantly scorning Jaune, deeming him a worthless failure for all he has done and failed to do.

Amongst them, his biggest inner demons have taken on the shape of Pyrrha and Penny, who speak of his name with disgust.

But worst of all is how the Nightmare Realm has developed a sense of continuity. Whenever someone on the team dies, when they return, the people in the Nightmare world (who are generally pulled from all across RWBY) acknowledge their fates and react to their previous actions. The world itself begins to evolve in response to the invaders, preventing them from carrying out any previous form of attack that they'd tried and forcing them to constantly come up with new strategies. New Grimm spectres and Nightmare threats are created with each defeat suffered by the heroes to compensate for their previous weaknesses. One try Team RWBYNR could be up against a simple horde of Beowolves, the next turn they could face variants of Grimm that feature humanoid forms, or can use weapons, or act as dark imitations of themselves to directly counter them.

(Honestly, whenever I picture the enemies that the Nightmare conjures, I always imagine the Dread from Destiny 2, so if I were writing this in full then I'd probably use them as a basis. I've already got an idea in my head of the Elementals, a group of Grimm humanoid-like soldiers based on the Attendant and Weaver Dread Psions, but with each variant being infused with a different form of dust, like fire and ice and wind and hardlight and so on, and each one having abilities related to their dust type.

In actuality, this idea can allow a writer to ignore any pre-existing rules that governed the laws of the Grimm in RWBY's canon. You can go ham on what type of Grimm you want to make within the Nightmare Realm with nothing to hold you back.)

All the while, as they fight and carve their way through the evolving terrors around them, through the ever-changing Nightmare, a voice in the darkness of Jaune's head.

Something within the Nightmare Realm is waking up. A personification of all Jaune's most negative traits, all his traumas and failures given physical form, pulling its way out of the horrors of the labyrinth and reaching out into the minds of the others, awakening their Nightmares and spreading itself out into themselves.

A creature, a monster that doesn't want to be a Grimm anymore...

It wants to be a human.

And it doesn't care who's body it has to steal, or who it has to kill to get there.