[Mad Pierrot]

Spoiler for spoiler:

Wait, we knew he was Kenjaku's creation some time ago but the dream also had a guy named Jin holding Yuji as a baby and I think Tsukumo said there was a woman named Kaori Itadori.

At least it explains why Yuji has so much power despite not training that much he said some chapters ago. I guess this might explain what are those red arms