Plot(Wikipedia)
A long time ago, a tribe that respected the tiger and controlled fire called the Ho-jok (Tiger Tribe), ruled the world. They eliminated and assimilated all tribes except the tribe that respected the bear, called the Ung-jok (Bear Tribe). The Tiger Tribe and the Bear Tribe fought, and Hwan-woong could not bear to see so many people getting killed. He brought the three gods of nature; Poong-Baek (백호, the White Tiger), Woonsa (청룡, the Azure Dragon), and Woosa (현무, the Turtle Snake).
He removed the power of fire from the leader of the Tiger Tribe, Kajin, sealed the power into an artifact called the Heart of Jujak (주작, the Vermilion Bird/Phoenix), and gave it to the woman he loved; Sae-oh. However, Kajin was mortally wounded while attacking another tribe, and she seemed to die until Hwan-woong healed her. This event makes Kajin develop love for Hwan-woong. When her tribe declared in a war council they would Hwan-woong take back the power of fire, Kajin then slew her tribe's elders with her own blade. She then escapes from her own tribe in search of Hwan-woong, deciding to become Hwan-woong's woman.
However, Hwan-woong likes Sae-oh and declared his love for her, not knowing that Kajin was watching them secretly when this took place. Kajin was infuriated by this and began reforming the Tiger Tribe and attacked the Bear Tribe. Unfortunately, this was the time when Sae-oh was pregnant, and when she had given birth, the baby was kidnapped by Kajin who offered Sae-oh a deal to trade her baby for the power of fire. When Sae-oh refuses Kajin, Kajin responds by dropping Sae-oh's baby from a cliff, leading to Sae-oh going out of control and releasing the Heuk-Jujak (Black Phoenix), even though Hwan-woong had saved the baby. (Note: Jujak is roughly the equivalent of Phoenix.) Hwan-woong's subordinates, the White Tiger, the Azure Dragon and the Turtle Snake fought the Black Phoenix, but they were no match for it.
Left with no choice, Hwan-woong used his Heavenly Bow to kill Sae-oh, the source of the power of the Black Phoenix, and managed to seal it once again in the heart of the Phoenix. Kajin committed suicide by jumping from the cliff because she now "had no reason to live in this world any more." Hwan-woong sealed the powers of the 'Four Gods' (The Phoenix, White Tiger, Azure Dragon, and the Turtle Snake) in certain artifacts and left them in the care of certain people, telling them that they will wake when the King of Joo Shin is born.
Many centuries later, Damdeok, the reincarnation of Hwan-woong, was born. Along with him, the artifacts of the guardians of the King were born. Damdeok met difficulties in becoming the king of Goguryeo, mainly because of Yeon Hogae who mistook Damdeok for the murderer of his mother, and his father who turned the king's parliament against him.
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