You have already gone past at least the initial encounter in your Jedi Fallen Order playthrough, so I think this is safe to say: Cal meets Night Sister Merrin when he visits Dathomir for the first time. She was a young child when this massacre happened and was the lone survivor of it - I mean, aside from Ventress and Mother Talzin, of course - Merrin was manipulated into believing that the Jedi orchestrated the massacre and were responsible for the killing of all her sisters.