Maybe I am misunderstanding something, but the progression of challenges seems to be broken after defeating the Naga, when Humaire gives me the obsidian knife.  When I click on the pool, the knife is in the artifact box but the only challenges listed are the two I have already completed.  I fought the Naga again, trying to advance the quest that way, but it seems like I cannot progress as intended.

 

I added Maia to my party at the "inn" after fighting the Naga and before talking to Humaire.

 

 

This might seem dumb, but have you used the blade to get watcher's blood?

If you have done that already, when the window open up after you click the pool, have you tried dragging the the blade and dropping it into the circular thing just beside that?