You had been sent as reinforcement.

Shibuya was under attack, a large scale attack led by the enemy, the one called Geto with his special grade curses and curse users and with them an army of transfigured humans, cursed spirits and sorcerers. It was chaos.

You made your way downstairs, manage to fight through and get to the lower floor where you see Satoru Gojo, facing Geto. Geto was about to seal the strongest sorcerer, Mahito being with him.