The defining twist of The East Palace reveals that the catastrophic supernatural curse plaguing the dynasty was never initiated by a random mythical demon, but was entirely orchestrated by human ruthlessness. Thirty years prior, the current King's biological mother, Royal Concubine Suk-bin Choi, systematically poisoned the legitimate sons of the late King to pave the way for her low-born son, Ju Sang, to seize the crown. To disguise her cold-blooded assassinations as a curse, she drowned witnesses and burned alive suspicious court ladies in the East Palace compound, creating the massive cluster of resentful spirits (wongwi) that formed the Pond Demon.
Furthermore, Saeng-gang's investigation reveals that her own mother, a former palace lady who possessed secret records of the poisonings, was slaughtered by the same matriarchal cabal, making Saeng-gang both the King's illegitimate half-sister and the daughter of his mother's victim. In the explosive finale, the king is forced to witness the terrifying reality that his entire reign is an illegitimate sham born of his mother's atrocities. Gu-cheon and Saeng-gang successfully neutralize the supreme spirit not by brute force alone, but by bringing the King to confess his dynasty's sins, allowing the silenced victims of the East Palace to finally pass peacefully from the mortal coil.