That quest always had to be completed before you save Halsin. Once he’s saved you can’t complete it.
Maybe there will be some alternative quest involving Kagha after EA for characters who didn’t figure out her plans and stop her. But for now, once Halsin comes back he stops the ritual and demotes Kagha, so she has lost her window of opportunity to bring the grove over to the Shadow Druids.
But that doesn’t mean she won’t try something else later on.
By the way, since you didn’t do the confrontation with Kagha, did Halsin make you Faithwarden and give you the special staff?