Aurelius is the human; Svadilfari is the leader of the Red Hammer tribe.
Leaving the cave, did you get the cutscene of Svadilfari taking flight?
If you fly near/by him with your Fire Breath going (which would be trivial if you could use Target Lock), does that do any damage? Even a little bit of damage could hopefully reset something and allow you to start targeting him.
If you re-enter the cave, maybe take out any remaining goblins, and go outside again can he be targeted then? If not, try going to your battle tower or Broken Valley, and then return to that part of the fjords.
