This happens after Class 1A start working as Heroes. But someone or something started prowling the streets at night... (I have only seen mha until season 3 so don't blame me if it's inaccurate ;-;) (Villain Tanjiro AU don't judge hehe)
*MANGA SPOILERS* A happy ending for attack on titan :)
Tanjiro(demon) was transported to the Jujutsu Kaisen Universe by a special grade curse called the Vynx. That's it. :/ (I want to finish my other fic before pushing myself to actually update 1/2 times per week)
After the Mugen Train Arc, Tanjiro was sent to another place for a mission instead of following Tengen Uzui To the Entertainment District. There he got attacked by Douma, Waxing Moon Two. He ended up in the Infinity Castle face to face with Muzan. But Muzan has other plans for Tanjiro... (Rengoku is still alive and Tanjiro is a demon. Rengoku is alive cuz I'm still sad ;-;)