The Breaking Teeth, Not Hearts is found at the camp overlooking the dock at Nyhofn, Vinland.
Follow the Toothbreakers
To access this quest, you’ll need to first complete the Flight of the Fancy mystery in western Vinland. When you speak to the man at the camp in Nyhofn he’ll ask you to join the Toothbreakers adventuring group on their mission to slay the beast in the woods to the west. You’ll need to follow them through the forest then slay the mountain lion that’ll eventually attack you. When you kill the lion, you’ll need to speak to one of the adventurers to complete the mystery.