When he beat Diarma’s ass in the mind-realm, he got Diarma’s core as a reward. From what I remember he later cut open his chest and shoved that core inside of himself (to incubate it?), hence the scar. Probably can’t heal it cuz it’s an L-class wound caused by a transcendant’s (Diarma’s) core lol
The amount of lore they gave in the side story to justify the mpreg lmaoo