The Serpentine's Underground Fortress is a location in the world of Ninjago. It is a massive subterranean lair, built beneath the Constrictai Tomb for the Serpentine to use during their hunt for the Fangblades. Numerous tunnels connect the fortress to the other Serpentine Tombs, enabling the snakes to move around Ninjago without being detected by their enemies.
The Ninja quietly move in the shadows of the Underground Fortress, watching the snakes converse among themselves. However, they are quickly spotted, and Nya tries to contact Lloyd to no avail. As all four Ninja fight the Serpentine, they notice that in the middle of the room is all four silver Fangblades. Cole makes a run for it, but is quickly stopped in place with a trap. Spikes rise up from the ground and create a giant cage, encasing them.
Nya successfully contacts Lloyd and informs him of their situation, to which Lloyd rushes over. Chokun and Acidicus suggest they celebrate, and Pythor agrees after a bit of convincing from Skales. The Serpentine proceed to celebrate with the Slither Pit, and while the Ninja contemplate an escape plan, Lloyd arrives, however, he is noticed immediately because he trips and falls all the way down to where a crowd of snakes are cheering. As Lloyd is humiliated, Lord Garmadon appears with the Skulkin army behind him, and they rush into battle. Lloyd releases the Ninja from their confinement, and they escape with the Fangblades.