#ideally this plot would also include copious amounts of steddie but thats up for interpretation lol

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Okokok so I've had this silly little idea trapped in my brain/jotted down in my notes for a LONG time and since my sister just reawakened my ST fixarion I think now is a good time to bring it to light: Kas!Eddie but not as a creation of Vecna or general Upside-Down weirdness, but as a vessel of the Mindflayer. More under cut bc this is definitely gonna be a long post lol

So, when it seemed safe enough to return without Vecna taking notice, due to his power being weakened in the fight, the Mindflayer returned, searching for a vessel it could hide in while regaining its strength. It found Eddie, and breathed life back into him by using the bodies littered around him to rebuild what was damaged, and give him a form more suitable for survival in the Upside-Down.

So, basically, my idea was that a tiny portion of the Mindflayer (particularly the one that escaped Will) managed to escape Vecna's control, but couldn't truly act on it until the rest of the hivemind was destroyed in Russia. The Mindflayer had learned a lot from its time under Vecna's control, through the vessels he had forced it to possess, about humanity. And it wanted revenge on the man that destroyed the land it once called home.

Eddie was the one to name it Kas after he woke up, using his new form to take shelter from Vecna's watch. It liked that name a lot better than the ones it had been given before, and over time its image evolves to look less like Billy, reflecting its growth into its own person. As they grow stronger, Kas is able to reach out to more of the inhabitants of the Upside-Down, connecting them to the hivemind and helping them escape Vecna's grasp. They all wanted him gone, just as Kas and Eddie did. They began amassing an army, subtly using their influence to help those that fell into the Upside-Down, guiding them to take refuge in the school. Eddie couldn't help them directly, but he and Kas could lead them to people that could.

It initially takes the form of Billy in order to communicate with him while his body was being rebuilt. It explains how Vecna— or The Invader, as those in the Upside-Down call him— had taken control over it, using it as a puppet to enslave the creatures of the Upside-Down and later Hawkins. It was conscious of everything, watching as its home was distorted and warped into something it didn't recognize, and then as it was forced to corrupt the minds of humans. It saw and felt everything they did, everything they were, and it grieved as they were slowly destroyed under Vecna's influence. It mourned the loss of this beautiful new form of life it had been forcibly made a part of. It explained to Eddie that it only wanted to help him fix it.

Jason Carver was the one they brought the survivors to, not knowing that he had died that night at the Creel house. He was Vecna's new puppet, along with the other bodies littering the Upside-Down, as Vecna built his army of the dead. He used Jason to influence the survivors, sending them on fruitless tasks that should have led to their deaths; but they somehow came back, chased away by a strange monster he didn't recognize. He never got a good enough view of it, but it somehow always evaded his control...

Eddie doesn't realize what was wrong with Jason until he's reunited with the party, and learns that Jason Carver is very much supposed to be dead. Then things start to make sense, and both halves of Kas are filled with dread at the realization that they had been putting the survivors right into Vecna's hands. They have to save as many as they can, but there's no way to tell how many had been corrupted, and they couldn't put their friends in danger to fix their mistake. Eddie and Kas have to find a way to save the remnants of a town that believe they're the monster, now for more reasons than one.

...

So yeah thats a summarized version of the story in my head, its all pretty loose but it takes a lot of inspiration from the D&D lore, but in a bit of a different light. I'm not sure if I'll ever do much with this concept, espescially since I don't have the attention span to write out a long, drawn-out storyline like this, so I might as well toss it out there for others to maybe take some inspiration from!

Please send me a link if you do, as I'd love to see what you do with it!

(Also sorry for the tense changes my brain is weird and thats just how it fits ig)