While in the final battle between Shigaraki and the hero's you landed a very good punch in Shigaraki's stomach. However after the impact some black tentacles came out and engrasped you pulling you into him. You blacked during the process.

After waking up you were greeted with the sight of Shigaraki and All for One slowly merging into eachother. There were hands everywhere. Next to the himunculus was a little boy with black hair. He was sobbing while trying to pull All for One away from Shigaraki.

"GET OFF HIM! I MEAN GET OFF ME!" He whimpered as he clawed AFO's face. But it kept regenerating All for one was laughing maniacally while the adult Shigaraki was pushing AFO away.

"Get off me! This isn't what I want!" Shigaraki yelled as he pushed away at the bit where AFO is connected to him.

It seems that there are three persona's in Shigaraki. AFO, Shigaraki's inner child and Shigaraki himself. They were all fighting to be on the surface but the kid was trying to help his older self. Maybe if you help one of them you'll be able to go go back to reality.