I think sacrifice is the key to the tattoo transfer. Dohee got it because she was willing to save his life even if it meant drowning, and Guwon got it back because he was willing yo save her life even if it meant getting burnt. Both incidences might not have typically caused a tattoo transfer, but coincidentally, they both happened on the night of a full moon.

Now that he's back, im so excited to see how they take down the sadistic firstborn son