you need to kill Thane in that time when Thane shout help by summit or lose to slime to access to the naga route (Thane's death is a little brutal(honestly is very brutal in my opinion))

if you want Thane death is little painless you should go lizard route (he will lay on that tree, the tree in the place when him and we talk about how to stop the war), he makes us keep our promises not to kill the rest of bull tribe have been escaped from the tribe. (it very sad and I cry every single time when I play this route)