No more details so far (ch.20). Taegun only said he hurt himself due to past trauma. Eunsung was too shocked to ask any more. Then Taegun left, so that is all. ☹
Omygosh, maybe, there was really a lover guide involve. What if that guy blamed Taegun for being so vulnerable then assaulted him? What if he even said, Taegun was practically seducing him even though he was just like that due to his powers? Which made him blame himself
Spoiler about the scars
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Tae Gun confessed that they are self-harm wounds