God of War Ragnarök

Missable!

This is earned when you first get the other favour of shutting down forges on Svartlfeim. On the big island in the centre (the one with sindri on it). Turn right whilst facing him and climb up towards the lighthouse. To the right of the top of the climb is a locked gate. Mimir will mention he has a prisoner locked behind there.

He'll ask you to get the key.

Side note: There is a miniboss that can drop useful crafting gear behind the lighthouse, if you wish to kill it now.

Head back to the boat, and sail towards the next forge. On the island to its right is a Dock. Dock there, you'll know if you're in the right place if Mimir mentions that this is the right island. Get the key and return to the lighthouse.

Open the gate and follow Mimir's instructions and the Lyngbakr will appear.
Get back in the boat and go to the Dock on its side. Now you must free the creatures fins and tail before boating up to its side and talking to it.

The trophy pops once you start moving away from the poor thing.