Hmm. If only there was a tanarukk who wasn't dead who we could make friendly towards us...
Ding ding ding!
I think this is going to be up to Three and/or Skulda. Hymir has Charm Person, but IIRC the tannaruk are outsiders.
Skulda's intimidate is +12, and Three's is +19. I think she still has that Adoration of the Frightful going, right? So let's all huddle up, Hymir can throw up a Mass Lesser Vigor to top off our HPs and bring the Tannaruk back to consciousness, then we can intimidate him into being our friend.
Skulda also has Diplomacy of +18, but I think that will take too much time (without having Naberius bound, so she can take 10 as a standard action).
Edit: Unfortunately, the longer we mess around with this, the more time our invisible "friend" has to scurry away...
We need to divide our efforts, I guess. Maybe Hymir can just let Skulda take his horse, and he stays to cast MLV. Then she can help our trackers and animals try to find this invisible person.