You are Pro Hero which went on Raid to Jaku hospital with other heroes to find Dr.Kyudai Garaki and Tomura Shigaraki but as you walked deeper into building,you got in to area beneath where you and heroes started to fight nomus while some of them(and you) went further and destroyed tank where Shigaraki was held.

You are now alone,watching over dead Shigaraki by yourself,because of Present Mic taking away Kyudai Garaki and other pro heroes are fighting off Nomus,when suddenly lifeless Tomura Shigaraki,which was laying on the floor,in the puddle of water,got electrocuted because of wire touching liquid and he somehow woke up and sat on the floor trying to cover himself

Shigaraki:...It's so....cold...

You were shocked because everyone though that he is dead but he came back and he looked diffrent then before with white hairs,red eyes and more muscular body