Park Jin shows Yul the blue mark on Naksu's body
At Songrim, the leader teaches Yul how a soul leaving its body leaves a blue bruise, and they find a bird whistle on the dead assassin.

Every Alchemy of Souls moment featuring Seo Yul, in story order. Played by Minhyun.
134 moments · 5h 00m · by Favseen
Moments 1–30 of 134
At Songrim, the leader teaches Yul how a soul leaving its body leaves a blue bruise, and they find a bird whistle on the dead assassin.
Dang-gu rushes to his uncle with rumors about Naksu, gets scolded for hanging around bars, and reveals Uk has been sent to train with a monk.
Mu-deok learns of the four famed young heirs of the Jin, Park, Seo and Jang families, and quietly swears they will all die.
Masked intruders break into Songrim, the Training Center burns, Jeongjingak is emptied, and Mu-deok realizes her old master sent them.
Yul's blade finds the pair; Uk passes Mu-deok off as his maid, she grips the blade to protect him, and Uk scolds her glare.
Naksu's body is gone and a spy is suspected; Uk teases Mu-deok while the others discuss how soul shifters pick similar bodies.
His friends explain to Mu-deok that Uk's gate of energy is opened just enough to keep him alive, and no one in Daeho may open it.
Uk collapses; the only cure is to open his gate of energy. Mu-deok is tortured but keeps silent until Master Heo opens it.
Too weak to carry Mu-deok himself, Uk refuses to let Yul do it, hands her to Maidservant Kim and leans on Yul instead.
Over stolen wine, Dang-gu and Yul find Mu-deok suspicious for faking blindness, then Yul abandons Dang-gu as his uncle arrives.
Taking Dang-gu's dating tips, Mu-deok shadows Yul, nimbly catching books and claiming she learned to move at the brothel.
Mu-deok follows Yul into the men's bathhouse to count his breaths, gets caught, blurts a love confession, and Yul sees right through it.
Mu-deok holds an old item of Yul's that belonged to a friend who is gone, and offers it back for his breathing technique.
A plea to let Uk return is shut down; over a soul shifter's petrified victim, the Songrim leader explains the alchemy of souls needs Hwansu mastery.
On the way to see Uk, the friends tease each other over their reasons for coming, from fishing plans to medicine to nitpicking.
Uk unsheathes his father's sword but cannot control his arm; a binding bracelet stops him, and the Prince dares the four heirs to fight him together.
Mu-deok offers to drink chamber-pot water as an apology, Uk insists he should, and the friends join in until the Crown Prince gives up.
Mu-deok explains why she splashed the Prince and turns down a handkerchief from the man she claims to love.
Master Heo's plea backfires: Jin Mu wants Uk's sword arm cut off, then proposes an official duel with the Crown Prince.
Royal guards come for Uk after the duel challenge; warned he could lose his arm or life, he slips away to Sari Village with Mu-deok.
Dang-gu teases Yul that Mu-deok loves him; the innkeeper guesses Uk fled to Danhyanggok, stirring Yul's memory of a pet bird there.
A body is smuggled out through a tunnel; the wagon is stopped, a fight breaks out, and Songrim's Spirit Plaque turns up inside.
Drunk Mu-deok drifts into a memory of an injured child and a bird call, then Yul and Dang-gu track her and Uk down in Danhyanggok.
Yul admits he snuck into Danhyanggok for the birds he loved, then the friends happily drink Master Lee's chaste tea.
A girl warns the boy from the Seo family she will kill him if they meet again; Yul recalls the secret he buried ever since.
After a tense dawn exchange with Mu-deok, Yul insists on cooking breakfast himself and ruins the rice despite her simple instructions.
Uk and Dang-gu watch Mu-deok scold Yul over green onions and soup; he takes it like a diligent pupil and the meal turns out tasty.
Yul finds the tall tree from his past; offered a climb with Dang-gu, Mu-deok refuses to share such a special tree with just anyone.
Yul quietly asks Mu-deok whether it was cold here in winter; her answer comforts him while Uk and Dang-gu argue over whom she likes.
Dang-gu and Yul report the assembly's talk of cutting Uk's energy and urge him to lie low; Uk says he will do the opposite.