- "Lucky for you southerners, the Free Folk rallied to a King-Beyond-the-Wall, as we will when need be, and marched on the ancient castle he'd taken for his own: the Nightfort. With the help of the Starks, we killed the demon and cleansed your precious watch. Then they thanked us, and kicked us back across the Wall, as you always have."
The Fall of the Night's King was a battle that took place in the Age of Heroes.
PreludeThe thirteenth Lord Commander of the Night's Watch found in the Haunted Forest a cold woman with bright blue eyes, seemingly a female White Walker. He brought her back through the Wall and declared himself "Night's King" and her his Queen. For thirteen years they ruled over his brothers in the Night's Watch, making human sacrifices.
BattleThe Free Folk rallied behind Joramun, the King-Beyond-the-Wall and marched against the Nightfort, which the Night's King had taken as his seat, defeating him with the aid of King Brandon the Breaker and House Stark.
The Free Folk were unceremoniously forced back Beyond the Wall and the Night's King and Queen were most likely killed for the crimes they committed, releasing the Night's Watch from their dominion.
In the books
In the A Song of Ice and Fire novels, the Brandon the Breaker (said to be the Night's King's brother, according to some alternate versions of the tale) and Joramun, the King-Beyond-the-Wall formed an alliance to defeat the Night's King and free the Night's Watch from his rule. After his fall, when it was discovered that he had been sacrificing to the Others (possibly in a similar way to Craster), all records of him were destroyed and his very name was forbidden.
After the defeat of the Night's King, the rule was enforced that the castles of the Night's Watch along the Wall should never be fortified against approach from the south, so that they cannot oppose the lands south of the Wall which they are meant to defend. The downfall of the Night's King also resulted in the strict enforcement of the rule that the Night's Watch is meant to be politically neutral, as guardians who do not "rule" the Wall but who serve the realms of men.