In the Ninjago story, they were called skeletons, but have also been referred to as Skulkins in places such as LEGO Battles: Ninjago. They are enemies of the ninjas. Samukai, a four armed skeleton and puppet King of the Underworld, uses his Skulkin army to help Sensei Wu's evil brother Lord Garmadon, the true King of the Underworld, to steal the weapons of Spinjitzu and use their power to reshape the world of Ninjago. Samukai once ruled over the underworld, but was deposed by Garmadon upon his banishment.
The Skulkin were once mortal warriors, but failed to either come to terms with or understand the fact that they had passed.
Skulkin also appear in LEGO Universe, which takes place after the main story of Ninjago's first year.
According to in-game scrolls in the initial Crux Prime update, after Kai, Cole, Jay, and Zane recovered the Golden Weapons, they returned to the Monastery. In pursuit of the weapons, the Skulkin attacked, and while the four ninjas were able to escape with the Golden Weapons, the Skulkin destroyed the walls of the Monastery with their machines. Sensei Wu then used his powers to steer the last remaining piece of the Monastery away from the world of Ninjago, and towards the last shard of pure Imagination on Crux Prime. However, the Skulkin followed close behind, and set up a mining quarry nearby the Monastery on Crux Prime. There, they found that they could use Maelstrom ore in the ground to protect themselves from Spinjitzu, and quickly went to work mining the rocks. Sensei Wu then sends the player on a number of missions to defeat the Skulkins, such as smashing the miners or destroying their drilling rigs.
Details of this storyline were later retconned after the Ninjago Monastery update, so that the Skulkin no longer destroyed the original Monastery. Instead, the Skulkin were sent by Lord Garmadon to Crux Prime after the Darkitect informed him about the Maelstrom's power. The Skulkin set up their Maelstrom ore mining operations on the planet chunk, but they were followed by Wu's faithful ninja Neido. The Skulkin retaliate by repeatedly attacking Neido's Monastery, but he holds them back by using Imagination Spinjitzu.
Later, the Ninjago Monastery world was introduced to LEGO Universe, in which the Skulkin were besieging the Monastery. Players had to destroy their equipment and Skulkin, then venture into the caves, where four named Skulkin had gotten hold of the four golden weapons.
A series of four Ninjago Dungeons, one for each element, were going to be released for LEGO Universe, with Garmadon as a boss at the end, but LEGO Universe was cancelled and the dungeons were never released.
- Frakjaw, the sentry who can spot the stealthiest of ninja
- Krazi, the crazy warrior who is truly mad
- Bonezai, the engineer and inventor of the vehicles
- Chopov, the mechanic hoping to be engineer
- Samukai, the former King of the Underworld
- Nuckal, a dimwitted but strong general
- Wyplash, a stealthy and intelligent general
- Kruncha, the former 2nd-in-command and currently the leader of the Skulkin
- Skeleton Miner
- Skeleton Engineer
- Skeleton Pit Boss
- Skeleton Watchman
- Skeleton microfigure
- Skeleton general microfigure
- Skull Truck-Belongs to Nuckal and Kruncha. It was a large-sized vehicle.
- Turbo Shredder-Belongs to the Skulkin Army. It was a large-sized vehicle.
- Skull Motorbike-Belongs to the Skulkin Army. It was a small-sized vehicle.
- Nuckal's ATV-Formally belonged to Nuckal, currently belongs to the Skulkin Army. It was a medium-sized vehicle.
- Skeleton Helicopter-Belongs to the Skulkin Army. It was a medium-sized vehicle.
- Skeleton Chopper-Belongs to the Skulkin Army. It was a small-sized vehicle.
- Turbo Turtle-Failed creation by Chopov. Its size was unknown.
- 2114 Chopov
- 2115 Bonezai
- 2116 Krazi
- 2257 Spinjitzu Starter Set
- 2520 Ninjago Battle Arena
- 2258 Ninja Ambush
- 2259 Skull Motorbike
- 2260 Ice Dragon Attack
- 2263 Turbo Shredder
- 2504 Spinjitzu Dojo
- 2505 Garmadon's Dark Fortress
- 2506 Skeleton Monster Truck
- 2507 Fire Temple
- 2508 Blacksmith
- 2509 Earth Dragon Defence
- 2518 Nuckal's ATV
- 2519 Skeleton Bowling
- 2521 Lightning Dragon Battle
- 30081 Skeleton Chopper
- 2254 Mountain Shrine
- 2173 Nuckal
- 2174 Kruncha
- 2175 Wyplash
- 3856 Ninjago
- Ninjago: Masters of Spinjitzu
- Pilot Episodes
- Way of The Ninja
- The Golden Weapon
- King of Shadows
- Weapons of Destiny
- Secrets of the Blacksmith
- An Underworldly Takeover
- Return to the Fire Temple
- The New Masters of Spinjitzu (Kruncha & Nuckal)
- Season 1
- Never Trust a Snake (Kruncha & Nuckal)
- Tick Tock (Kruncha & Nuckal)
- The Royal Blacksmiths (Kruncha & Nuckal)
- All of Nothing
- Day of the Great Devourer (Kruncha & Nuckal)
- Season 2
- Double Trouble (Nuckal)
- Ninjaball Run (Kruncha & Nuckal)
- Wrong Place, Wrong Time
- The Last Voyage (Samukai, Kruncha & Nuckal)
- Pilot Episodes
- LEGO Battles: Ninjago
- 55000 LEGO Universe
- LEGO Ninjago: Nindroids
- LEGO Ninjago: Shadow of Ronin
- LEGO Dimensions
- All of the Skulkin have torso armour, but in the non-spinner sets Krazi and Bonezai do not, and Frakjaw, Krazi, and Chopov also have their own special headgear.
- The skeleton in the Pirates of the Caribbean set 4195 Queen Anne's Revenge has a Skulkin torso.
- Except for Wyplash and Samukai, all of the Skeletons make cameos in Season 1 of Ninjago: Masters of Spinjitzu.
- The Skulkin appear as antagonists in Tommy Andreason's fan fiction story Way of the Departed, in which Clouse becomes their leader after ending up in the Underworld. The story also indicates that the Skulkin were once a horde of warriors that terrorized Ninjago before becoming skeletons and ending up in the Underworld.
- Since the name "Skulkin" was not officially approved, the Skulkin are generically referred to as "skeletons" in most Ninjago media and in LEGO Universe. However, the name is still used in some official sources, such as 71730 Epic Battle Set: Kai vs Skulkin and Be Ninja: The Skulkin Wipe.