You can't even process to begin this. What happened? Why did it happen? Could've it been Prevented?

"EN. CHAIN."

That's what Ryomen Sukuna said on Yuji Itadori's Face before taking controlling, ripping off his own finger after imbuing all of his cursed energy into it before force feeding it to Megumi Fushiguro and incarnating into the body of Megumi, gaining the Ten Shadows with the usage of Shrine Ontop of it.

Everyone was utterly devastated, with Satoru Gojo still sealed and the final 5 fingers of Sukuna out there and his Heian Body, practically noone could stand up to him. Aoi Todo and Nobara Kugisaki weren't there either being a massive Downfall for Yuji especially.

But there was atleast some chance of bringing back atleast Megumi with the help of Hana to Break open the Prison Realm and free Gojo, but untill then, everyone was utterly Devastated because the person with the most potential was taken control of. So many dead because of the Culling games, Shibuya Incident and Pre-Historical wars. There will be even more deaths during the Shinjuku Showdown. Until then everyone was away doing their own thing to pass the time and try grieve less for Megumi.