Tale of the Nine Tailed — Ending Explained

Ah-eum's True Death (600 Years Ago): Ah-eum did not die from a natural tragedy; she willingly offered her body to the Imoogi to save her possessed father. She then forced Lee Yeon to kill her so the serpent monster inside her would die.

The Dual Imoogi: The Imoogi was not just one entity. It split into two parts: the resurrected young male form (Lee Tae-ri) and a dormant soul fragment hidden inside Nam Ji-ah since her childhood car accident.

The Ultimate Sacrifice: Because killing the main Imoogi would require killing Ji-ah, Lee Yeon tricks the Imoogi by ingesting its soul fragment, drawing the entire monster into his own body. He then commands Rang to slash him and jumps into the Samdo River, dragging the Imoogi into eternal oblivion.

Lee Rang’s Redemption: Devastated by his brother's death, Lee Rang meets the Underworld Fortune Teller (the Lord of the Great Bear). Rang trades his remaining life force so Lee Yeon can be reincarnated. Rang bids a tearful farewell to his surrogate family before fading away.

The Climax & Epilogue: Yeon returns to Ji-ah in human form, and they hold an intimate wedding ceremony. In the final moments of Episode 16, Yeon encounters the Spirit of Smallpox at night, draws his red umbrella sword, and his eyes flash with gumiho light, revealing he still possesses supernatural guardian powers.

Interpretation A: The Pure Human Ending
Yeon sacrificed his gumiho immortality completely, returning to earth as a plain mortal human. He marries Ji-ah and lives a finite human life, fulfilling his dream of experiencing mortal love. The final scene where he hunts the Spirit of Smallpox with his umbrella sword is interpreted as a symbolic metaphor or a brief, final residual duty.

Interpretation B: The Secret Gumiho Protector
Yeon retained (or regained) his supernatural gumiho powers upon returning from the Samdo River. However, to give Ji-ah a peaceful, normal life, he keeps his powers secret from her while secretly slipping out at night to fight evil spirits as the city's hidden guardian.

Interpretation C: Cosmic Equilibrium / Interdimensional Deal
As a former mountain god, Yeon's divine nature can never be erased. His return is a special compromise arranged by the Lord of the Underworld and Taluipa in exchange for Lee Rang's soul sacrifice, allowing Yeon to exist as human by day and gumiho by night.