The Underworld is a dark realm where deceased lifeforms can become animate. Certain locations in Ninjago can transport people to the Underworld, the most notable one being the lava in the Fire Temple. The Underworld is home to the Skulkin, who share the realm with the Spykor, which live along the ceiling stalactites.
Pieces of land stay afloat in midair, connected by long wooden bridges that the Skulkin regularly traverse over. They keep torches lit by each bridge to provide sufficient lighting.
At an unknown point in time, the Underworld came into existence alongside the rest of the other Sixteen Realms, where it would function as the afterlife for fallen warriors and animals who failed to realize or accept their passing, trapped in a state of limbo.
Afterwards, Samukai would later die and end up in the Underworld, becoming its king in the process.
Following his banishment, Garmadon would swear vengeance and make plans to get the Golden Weapons by using the Skulkin as an army. Clashing with Samukai, he promptly won, declaring himself King of the Underworld as a result.
Master Wu awoke in the Underworld after traveling there via the Fire Temple's lava. He was forced to stay hidden as the Skulkin patrolled the Underworld, but snuck his way around them and headed toward Lord Garmadon's Dark Fortress.
Meanwhile, upon recruiting the Dragons, the Ninja relied on their new companions to travel to the Underworld. The Dragons quickly swooped down toward the land below, only to spin themselves into tornadoes which transported them through a swirling vortex. They then navigated through a hidden tunnel before finally arriving in the Underworld.
Kruncha and Nuckal were guarding the Dark Fortress, inspecting any incoming objects to the building. Both Skulkin Generals were unaware of the Ninja, who were traveling quietly by stalactites. However, this plan failed as they found themselves mistaking the legs of Spykor for stalactites and fell in front of Nuckal and Kruncha. After Nuckal alerted his brethren to the intruders, the Skulkin began cornering their opponents as several Spykor dropped from the ceiling to join them.
While the Ninja began tossing ideas back and forth for their next move, Jay suggested the Tornado of Creation. Each Ninja whipped into a Spinjitzu tornado, all four combining into one giant whirling mass. The Skulkin and Spykor were pulled into the massive tornado, and ended up trapped in a skeleton ferris wheel. Taking this as their cue to find their master, the Ninja immediately headed into the Dark Fortress.
Whilst discussing the concept of the Sixteen Realms on a floating gondola, Fenwick showed the Ninja an illusion of the Underworld.
Fighting over the Realm Crystal, Morro and Lloyd would arrive in the Underworld. After Lloyd was attacked by Spykor, Morro would attempt to leave him behind through a portal, only for the Master of Energy to follow in pursuit.
- So far, it's the only Realm with the most entrances to it: death after being dishonored in battle, Dragons, a physical entrance behind the lava fall in the Fire Temple, the Skulkin's vehicles (when driven at a very high speed), and the Realm Crystal.
- The Underworld was the first alternate realm seen in the TV series.
- Oddly enough, set descriptions for 2013 sets claim the Stone Army come from the Underworld.
- In the show's early years, the Underworld was referred to by supplementary material as the place where all beings go when they die, as an "underworld" traditionally is in fiction. This was retconned however, being that in the current story, the Departed Realm exists as seemingly the true place beings go after they die.