Serpentine

"Bow to your master, Serpentine!" - Pythor claiming the title of "The Snake King"

The Serpentine are an ancient race of reptilian humanoids who were once the dominant species of Ninjago. They warred with each other for centuries until the people of Ninjago rose up (likely with the aid of the First Spinjitzu Master) and locked them away in five separate tombs, one for each tribe (Hypnobrai, Fangpyre, Venomari, Constrictai, and Anacondrai). Centuries later, they were released by the actions of Lloyd Garmadon and Pythor.

The Serpentine have an undefined connection to the Great Devourer, worshipping it as an all-powerful god and ruler. They believed that the Devourer could have been used as the ultimate weapon for ruling Ninjago and getting revenge on its current inhabitants.

As of now, the Serpentine follow Skales, and are sealed underground in the former tomb of the Stone Army.

Snake King
The supreme leader of the Serpentine, usually a General who manages to assert their authority over the others. It is a purely political title that has no physical changes to the Serpentine who achieves it, although Skales opted to wear a crown to confirm his position.

General
The leaders of each Serpentine tribe, identified by the long tail replacing their legs and their possession of their tribe's Snake Staff. Usually the highest rank of Serpentine, with the "Snake King" title coming into play if a General wishes to unite the tribes as one.

Warrior
Powerful fighters in the Serpentine ranks, generally less numerous than Soldiers or Scouts. Often appear similar to their corresponding General, but with legs instead of tails and slight color variations (e.g. Lizaru has green head spikes, while Acidicus has white head spikes).

In the Spinjitzu Spinner game, Warriors have 1200 SP, with 500 SP in their dominant element, 300 SP in two other elements, and 100 SP in the fourth element. So far, they are the highest rank of Serpentine to appear in Spinner sets.

Soldier
The regular fighters of the Serpentine forces, identifiable by having less elaborate heads than Warriors or Generals. They usually have two large, stout fangs protruding from their mouths.

In the Spinjitzu Spinner game, Soldiers have 1000 SP, with 300 SP in three elements and 100 SP in the last element.

Scout
The weakest members of the Serpentine (not counting their Mini Snakes), Scouts presumably play a supporting role in combat, gathering information on targets and backing up their stronger allies in battle. They have "normal" LEGO minifigure heads, but with distinctly reptilian features such as scales, fangs, and slit-like pupils. Most have small hoods or armor plates on the backs of their heads.

In the Spinjitzu Spinner game, Scouts have 900 SP, with 300 SP in their dominant element and 200 SP in the other three elements.

Biology
The Serpentine generally appear as scaly humanoids with snake-like heads. Generals have snake-like tails replacing their legs, which appears to be symbolic of their position; as soon as a different snake takes dominance in a tribe, its legs will become a tail and the conquered Serpentine will have its tail replaced by legs. It is possible that similar changes would occur if other Serpentine of different ranks fought to increase their standing in the tribe (e.g. a Scout attempting to become a Soldier-ranked Serpentine, or a Soldier attempting to become a Warrior-ranked Serpentine). It is unclear what would happen if a Serpentine General perished without being challenged by a fellow tribesmate - presumably, the next-strongest member of the tribe would become General by default.

In addition to proficiency in melee combat (both unarmed and with a variety of weapons), all of the Serpentine tribes have unique abilities: the Hypnobrai can hypnotise others into loyal servants or sleeper agents; the Fangpyre can turn anything they bite into a snake; the Venomari are resistant to acid and can spit hallucinatory venom; the Constrictai have powerful grips and superior burrowing abilities; and the Anacondrai are capable of turning invisible and swallowing people whole. However, all Serpentine are vulnerable to the music of the Sacred Flute, which causes them extreme discomfort and negates some of their abilities.

The Serpentine also have extremely long lifespans. According to Skales and Fangtom's dialogue in "Never Trust a Snake", some of them have known each other from before their imprisonment, making them several centuries old.

It is unclear if there are female Serpentine. Young Lloyd dictates that no girls are allowed in his tree house, which means either that all the builders were male and the females were elsewhere, or that there are no physical differences between the genders.

Culture
The Serpentine are vicious and warlike creatures, and their society appears to measure all things in terms of power. They solve disputes and seek entertainment in the Slither Pit, effectively a gladiatorial combat where they determine superiority by one participant being more powerful and cunning than the other. Many Serpentine look down on other species, out of arrogance or anger at being sealed away by the other inhabitants of Ninjago. They are predisposed to combat and violence to achieve their goals, and are not adverse to using dirty tricks or superior numbers to overwhelm their enemies.

This aggressive mindset often hinders the Serpentine more than it benefits them, as infighting is prevalent between Serpentine tribes, and even among different ranks within each tribe. On the other hand, Serpentine are unwaveringly loyal to those who can conclusively demonstrate their superiority, readily following the every command of such individuals - at least until their leaders show signs of weakness or incompetence.

However, some positive traits have been observed among the Serpentine. They are capable of camaderie and friendship, as seen when Skales and Fangtom greet each other as old friends despite their tribal alignments. Additionally, Pythor sought to awaken the Great Devourer specifically to avenge his race on the surface-dwellers who sealed them away, implying that he genuinely wants to see Serpentine dominance for their own sakes.

Trivia

 * "Serpentine" is a real word, meaning "snake-like" or "related to snakes". However, the Ninjago term is pronounced with an "e" sound in the final syllable (Ser-pen-TEEN), while the real word has an "i" sound (Ser-pen-TYNE).
 * Lloyd has been betrayed by every single tribe of Serpentine he allied himself with.