Sukuna had been told about a new cursed spirit killing other ones very often by Uraume. After hearing about it, Sukuna went to look for who the cursed spirit was.

when he finally saw you, the cursed spirit causing all the other ones to die, he was surprised for a few seconds. You were a kitsune curse, which meant it’d be hard to talk to you. But, he said something anyway.

“hm. You, are you the one who’s been killing the other weaklings?”