|“I present to you, the city formerly known as the Lost City of Ouroboros!”|
|— Pythor to Skales|
The Lost City of Ouroboros, formerly known as the Forsaken City, is a massive Serpentine city which was buried beneath the Sea of Sand after the Serpentine War. The Great Devourer was imprisoned underneath the city, and consequently, finding Ouroboros was a key element of Pythor and the Serpentine's plans for vengeance against the people of Ninjago. The city was originally used as the Serpentine's base before they set off to claim the Fangblades.
After retrieving the four Fangblades, Pythor returned to Ouroboros and began awakening the Great Devourer. However, Wu arrived and confronted Pythor just before the Great Devourer rose from the city and consumed them both. As a result, Ouroboros was heavily damaged, yet this didn't stop the Serpentine from using it as their base again, yet once more it was only temporary. The city was abandoned after the Serpentine journeyed to the Stone Army Tomb.
During the Nindroid conflict, Pythor brought an army of Nindroids to Ouroboros, which was briefly used as their base of operations. While at the city, the Nindroids constructed and launched a giant rocket into space, en-route to a comet that held the remnants of the Golden Weapons.
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.
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.
During the hunt for the Fangblades, 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.
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.
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.
After the destruction of the Mega Weapon, the Serpentine returned to Ouroboros where they resumed hosting Slither Pits in the main arena. While Lasha was fighting Lizaru, Garmadon arrived in a Rattlecopter to gather the Serpentine Generals in order to search for the Island of Darkness. They soon departed in a Rattlecopter over the Endless Ocean, only for Garmadon to be pushed out into the ocean by Skales.
With Ouroboros as their primary base again, Skales was crowned Snake King over the Serpentine.
The city of Ouroboros appeared to have been Pythor's base of operations prior to his return to New Ninjago City, as he led the Nindroid army to it after retrieving the Overlord's body from the Nindroid MechDragon. The villains subsequently used the city to launch Arcturus and hold Kai captive.
In LEGO Ninjago: Shadow of Ronin (noncanon)
- 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 Rise of Snakes.
- 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."