Serpentine War

The Serpentine War is a conflict that took place following the first defeat of The Overlord and his Stone Army by the First Spinjitzu Master. It was fought between the five tribes of Serpentine and the humans of Ninjago, with the Anacondrai and the Elemental Masters serving as leaders of the respective sides. It ended with the defeat of the Serpentine, but was later reignited-first by Lloyd Garmadon, and then later by the Anacondrai Cultists.

Rise of the Snakes
Lloyd Garmadon, in his bid to force the people of Ninjago to respect him as the successor to his father Lord Garmadon, threatened to unleash the Serpentine upon them. The Ninja noted that the Serpentine had long ago been sealed away in ancient tombs.

The Curse of the Golden Master
Seeking answers regarding the stranger helping the Digital Overlord, the ninja visited the Serpentine in their new home beneath New Ninjago City. There, they were told the Legend of the Golden Master, a monstrous being whose coming had been foreseen by the Serpentine. Seeking to avert this crisis, the Serpentine tried to warn the humans, but were perceived as attacking them and driven underground.

Codename: Arcturus
After learning that Pythor and the Digital Overlord were working on Project: Arcturus, Garmadon and Sensei Wu recalled defeating a Serpentine General by that name.

Spellbound
While searching for Nya, Garmadon related to Lloyd his history with Master Chen during the Serpentine War. The conflict was a desperate one, made more so by the presence of the Anacondrai under Arcturus. Eventually, under the leadership of Wu and Garmadon, the Elemental Masters formed an alliance that saw some victory against the Serpentine-however, Chen managed to turn some of the masters against the others. Fortunately, they found their trump card in the Sacred Flutes, which they used to halt the Serpentine and drive most of them into their underground tombs. Arcturus and the other Anacondrai generals were then magically banished to the Cursed Realm. However, upon finding Nya, Garmadon learned that Chen intended to renew the war by transforming himself and his warriors into Anacondrai.

The Day of the Dragon
Chen and his forces succeed in casting the transformation spell, turning them-along with Skylor and Garmadon-into Anacondrai. After soundly defeating the new Elemental Masters, they escape to Ninjago.

The Greatest Fear of All
It is discovered that the new Anacondrai need the "essence" of a true Anacondrai to make their transformation permanent. Hoping to find a means of defeating them, the Ninja visit the last true Anacondrai, Pythor, at Crypterium Prison. During their interview, Pythor reveals that the truce both sides had negotiated during the war was sabotaged by Chen, who told each side that the other was planning to attack. Having learned of Pythor's existence, the new Anacondrai attack the prison and capture him, and manage to use his sweat to make their transformations permanent. They then launch their first offensive, distracting the new Elemental Masters away from populated areas to allow them to invade unchallenged.