The cloak mark.

The Potters. Descended from the Peverells and entirely pureblooded. Their most favored heirloom, the invisibility cloak. James Potter had inherited it from his father, and so on back in time. In this story, anyone who inherits the invisibility cloak will have the triangle mark form on his back after 17, the mark was taken from the Deathly Hallows symbol of the cloak. So either way, this is when James is 18. He’s sleeping shirtless, next to Lily, who’s eyes catch on her husbands mark which she runs her finger over. It’s a black mark, which has a texture of its own and oddly satisfying.