The Skulkin featured prominently in Ninjago's 2011 sets, as well as the pilot episodes of LEGO Ninjago: Masters of Spinjitzu. They were the first enemy faction in LEGO: Ninjago, being succeeded by the Serpentine in 2012, the Stone Army in 2013, the Nindroids in 2014 and the Anacondrai Cultists in 2015.
The Skeleton Army are skeleton minifigures with new torso, leg, and arm pieces. They wear belts and armor colored red, blue, gray, or white (their elemental colors). Generals have much larger heads then regular Skulkin, with different details on their scalps; Nuckal has a row of spikes, Kruncha has a helmet, and Wyplash has a stud to attach headgear (although the straw hat fits best).
The Skulkin have torsos with bolts printed above the rib cage. These torsos are new pieces with a colored belt depending on element.
The Skulkin have two types of arms, regular arms for holding weapons normally, and sideways arms for holding weapons out to the side for Spinner battles. Both types of arms are bent at the elbows.
Unlike previous skeleton figures, Skulkin have black shoes, mainly so they can stand on spinners.
The Skulkin wear different types of armor depending on their rank.
The trooper armor has bolts on the top and has metal across the chest with a Skulkin symbol. The sides have spikes over the shoulders that are colored differently depending on element.
The Generals have armor that feature the Skulkin symbol and bones connected by straps to the shoulder guards. The shoulder guards have colored spikes to represent elements (the only colors shown are gray and white because only Kruncha and Wyplash have them).
Due to his unique design and high rank among the Skulkin, Samukai has special armor. In the pilot episodes, it is brown, but in "Wrong Place, Wrong Time" it is white.
In the cartoon
In LEGO Ninjago: Masters of Spinjitzu, most Skulkin were based on Chopov's design, but with Lightning armor. Bonezai, Frakjaw, and Krazi made more sporadic appearances, likely because their more unique designs would draw more attention to the recycling of character models to fill out the Skulkin ranks (a problem that was more visible among the Serpentine).
The Skulkin wielded a variety of weapons, but not to the same extent as their set appearances. They usually wielded bones, bone axes, and their fists, but occasionally carried spears and dark swords. In "Wrong Place, Wrong Time", they also displayed the ability to remove their own arms and wield them as nunchucks.
The Skulkin Generals exhibited marked changes from their depictions in other media, with Kruncha becoming brutish and hotheaded and Nuckal becoming childish and unintelligent. This was likely to provide comic relief and to play up Samukai and Lord Garmadon's roles as villains.
The Skulkin army largely disappeared from the show once Season 1 began, with only Kruncha and Nuckal making frequent humorous cameos. However, they returned in force in "All of Nothing," where they were summoned by Lord Garmadon to storm the Serpentine's underground fortress and aid the Ninja against the snakes. When Pythor questioned why the skeletons were aiding Lord Garmadon, Kruncha replied that they hated snakes just as much as anyone. The skeletons proceeded to battle the Serpentine, holding them off long enough for the Ninja to escape with the Fangblades.
In LEGO Universe
The Skulkin also appear in LEGO Universe, where they attack the Monastery of Spinjitzu and send it into space. The Monastery crashes on Crux Prime, where the Skulkin happen to be mining Maelstrom Ore for unknown purposes. Seeking to investigate the skeletons' activities, Sensei Wu sends the Imagination Ninja, Neido, to monitor the Skulkin and stop their plans.