The Stone Army Tomb is a large structure underneath New Ninjago City. Following his battle with the Overlord, the First Spinjitzu Master vanquished the evil spirit and imprisoned his warriors, the Stone Army, in an underground cavern. They remained there for many years, untouched and dormant, until the Serpentine stumbled across the tomb. The Stone Army was awakened by the Great Devourer's venom, and trapped the Serpentine underground.
The Serpentine used their imprisonment to take on a new lifestyle, becoming peaceful and eventually aiding the Ninja in stopping the Golden Master. After his defeat, the Serpentine returned to the surface and lived among the people of Ninjago.
After the Great Devourer was killed, its venom began leaking into the ground. The Serpentine were united under Skales and schemed to burrow underneath Ninjago and cave in the city. However, they came across the entrance of the tomb. Upon entering, they found the Stone Army, which was revived by the Devourer's venom. The Serpentine battled the Stone Army before being defeated and locked up in the tomb, while the Stone Army set off to aid their old master.
Over time, the Serpentine adapted to the tomb and made it their new home. At this point, they turned away from their barbaric tendencies and tried becoming more civilized and peaceful.
When the Nindroid Conflict began, the Ninja journeyed underground to find the Serpentine, seeking to know more about the Golden Master. Eventually, the Ninja came across the tomb, and were confronted by Skales, who introduced them to their new life under the surface. Acidicus told the Ninja of the curse of the Golden Master, before Cryptor arrived with the Nindroids and attacked the Serpentine. The Ninja protected their former adversaries, shutting down most of the Nindroids. When they asked the Serpentine to help them in their quest, Skales told them they wished to stay out of the conflict. However, when the Overlord became the Golden Master, several of the Serpentine left the tomb and helped the people of Ninjago escape the ensuing war in the city.
After the Golden Master's defeat, the Serpentine were accepted into Ninjago's society, and abandoned the tomb in favor of living on the surface.