As the name suggests, 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 Lloyd Garmadon and Pythor.
The Serpentine have an undefined connection to the Great Devourer, worshipping it as an all-powerful god and ruler. They formerly believed that the Devourer could have been used as the ultimate weapon for ruling Ninjago and getting revenge on its current inhabitants, but they saw him failing.