The Lost City of Ouroboros

"I present to you, the city formerly known as the Lost City of Ouroboros!" - Pythor to Skales

The Lost City of Ouroboros is a location in LEGO Ninjago: Masters of Spinjitzu. It is a massive Serpentine city which was buried beneath the Sea of Sand after the snakes' first defeat. The Great Devourer was said to be imprisoned in the city, and thus finding Ouroboros was a key element of the Serpentine's plans for vengeance against the people of Ninjago.

Background
Pythor and Skales searched the desert for the Lost City of Ouroboros, and eventually unearthed it by turning a dial buried deep underground. With the city discovered, Pythor summoned the other Serpentine to the city's Slither Pit arena and challenged the four Generals for their staffs. With some help from Skales, Pythor successfully defeated the Generals and united the tribes, shortly before Lloyd Garmadon - spying on them in a Rattla disguise - accidentally revealed himself.

Taking Lloyd prisoner, Pythor manages to capture Samurai X and the Ninja when they arrive, before pitting them against each other in a Slither Pit. However, Samurai X helped the Ninja escape, defeating a few Serpentine and escaping with the Golden Weapons, although Lloyd remained a prisoner. (The Snake King)

Pythor's true interest in the Lost City of Ouroboros was its role as the apparent prison of The Great Devourer; consequently, he began searching for the four silver Fangblades, the keys to releasing the Devourer. (Tick Tock) During this period, the Serpentine largely abandoned the city of Ouroboros, opting to instead create a network of tunnels and a large underground fortress. When Pythor finally collected the four Fangblades, he immediately proclaimed that the Serpentine must return to Ouroboros to awaken the Great Devourer, but the Serpentine insisted on celebrating their imminent victory. This would prove to be a mistake, as it gave the Ninja time to steal the Fangblades and head for Torchfire Mountain to destroy them. (All of Nothing)

However, Pythor snuck aboard the Destiny's Bounty and stole the Fangblades back, immediately heading for the Lost City of Ouroboros. Despite the Ninja's best efforts, the Anacondrai General returned to the arena and used the Fangblades to reawaken the Devourer, and the city was heavily damaged as the monstrous serpent broke free of its prison beneath the arena. (The Rise of The Great Devourer)

Following the Great Devourer's defeat at the hands of Lord Garmadon, the Serpentine used a desert cave as a temporary base, as Ouroboros was apparently too badly damaged to live in. However, some time later, the Serpentine repaired it enough to use as a staging ground for their forces. When the Ninja disrupted the preparations, Lord Garmadon used the Mega Weapon to travel back in time and negate their existences. As a result of his meddling, the city of Ouroboros began sinking into the sand - as the Ninja had never been formed, the city had never been found. Presumably, it was uncovered when the Mega Weapon was destroyed and the past was restored. (Wrong Place, Wrong Time)

Trivia

 * Ouroboros appears to be the only Serpentine city.
 * Ouroboros means "snake or dragon biting its own tail" in the Greek language. This is referenced when the Great Devourer is defeated in a similar manner, as well as in the various spiral and circle designs around the city.
 * The Lost City of Ouroboros is a playable place on LEGO Ninjago: Rise of Snakes (App).
 * 9450 Epic Dragon Battle is supposedly Ouroboros.
 * There is a typo of the spelling of the city in the Ninjago Game App, where it is spelled "Aura Boris."