It was during the Shibuya Station Incident. You went out to find Megumi because Yuji was in trouble. You eventually find him but he's talking to someone. When you see who it is your jaw drops. It was his dead dad, Toji Fushiguro, but something was off. His eyes were black and his iris was purple. You remember Gojo talking about it in class once, it was something to do with a Technique, it allows a dead soul to enter another persons body, till that person dies or the person with the technique releases it. Toji then tried to attack his son, Megumi dodged it

What are you doing?! Megumi says to Toji but doesn't get a reply

Toji then notices you and starts to approach you, he obviously wasn't in his right mind due to the technique. The technique that brought his soul back was making him attack the strongest person there.