I think the scene where he shoots the star from the sky after three tries is likely relevant. I don't have the game pulled up, but I believe he does this with Phroneos although he gives the feathers to Androgeus. In that act and the underground labyrinth, there's a sense of brotherhood, family, and strength all tied together. The wrestling feels ritualistic because of it but is also a bit odd in that he's seemingly honoring himself, or rather a self he considers dead die to his supposed cowardice. I imagine it appears when he's subconsciously accepted his true nature. It's almost certainly tied with what Robert said. The gods need worshippers to exist, after all, so by honoring the piece of himself that is divine with your character, he may have inadvertently reawakened it.
Okay, that’s cool as hell. Plus, I think the birthmark also comes back when he’s decided for himself that he’s innocent, just before the renaming ceremony? He puts on the cloak and goes in front of the mirror, where he has the birthmark again. After this point, it doesn’t go away. I think it fits in really well with the idea of Asterion having faith in himself causing the birthmark to reappear.