Samukai

"No! You will obey ME now!" -Samukai beginning to betray Lord Garmadon.

Samukai was the formal King of The Underworld and Lord Garmadon's head general. Kruncha took his place after he was betrayed by Lord Garmadon.

Usurpation
Samukai was once the King of the Underworld until Garmadon was banished to the Underworld after losing to a fight with his brother Sensei Wu. Samukai, addressing his authority, attempts to subject Garmadon to a lifetime of pain and suffering for his wrongdoings, but Garmadon used Spinjitzu to engage into a fight with Samukai. After the duel ends with Garmadon finally defeating Samukai, Garmadon then took Samukai's helmet for himself, dubbing himself as the new King of the Underworld, much to Samukai's discomfort. Samukai then became relegated as the Skullkin General of all elements

Assault on Four Weapons Blacksmith Shop
Years later, he lead an assault on the Four Weapons blacksmith shop to find the map to the four Golden Weapons. He fought Kai one on one for a small amount of time before subduing him, but before he could finish him off, Sensei Wu interviened. Samukai then used his daggers to make a large water tower begin to fall on Kai, leaving Sensei with a choice: Rescue Kai, or capture Samukai. Samukai used the distraction to capture Nya and escape in his skull truck.

Seeking the Golden Weapons
He then went to find the first Golden Weapon, the Scythe of Quakes, hidden in the Caves of Despair. However, Samukai read the map upside down, and was digging in the wrong place. After the four ninjas gave away their presence accidentally, he took his army in to fight them and claim the weapon. Soon after the ninjas discovered Spinjitzu during the battle, he called a retreat, as he could not fight four Spinjitzu masters. Garmadon then gave Samukai new orders: Let the ninjas think they're winning. Samukai continued to seek the golden weapons, but allowed the ninjas to dominate until just before they began to search for the Sword of Fire. Then, Garmadon's scheme came into play. Kai was lured away from the rest of the group, and Sensei Wu followed. Samukai and his army then pounced on the other three ninjas, claiming the Shurukens of Ice, the Scythe of Quakes, and the Nunchucks of Lightning for Garmadon.

Betrayal To Lord Garmadon
Garmadon then sent him back to the underworld, where he fought Sensei Wu, who was wielding the Sword of Fire. Samukai defeated him, and took up the Sword, intending to use the four weapons and their power to defeat Garmadon and regain control of the Underworld. However, the power of the four weapons united was too much for anyone to handle, even Garmadon himself confessed that he himself can't control the combination of the powers. As a result, Samukai screams in agony as he disintegrates and a vortex through space and time appears, which was Garmadon's true plan all along. Having succeeded in his plan, Garmadon then escapes through the vortex, vowing to return with more power so that he can wield the four weapons and recreate Ninjago in his own image.

Appearance
Samukai had four arms and a large head. He also had two scars above his eyes, and wore the standard chest plate of a fire-based general of the Skeleton Army. He once wore the helmet Lord Garmadon wore in 2011, this helmet seems to go to the ruler of the Underworld meaning that Kruncha might wear it now.

Weapons
Samukai used four daggers. He also used all of the Golden Weapons at one point and was destroyed by them, because no one (except the First Spinjitzu Master and later, apparently Lord Garmadon) can wield all four at once.

Trivia

 * Samukai's name is a pun on of the word "Samurai"
 * Samukai is the only Skulkin that doesn't have a spinner/character card.
 * He is the first character in Ninjago to have 4 arms.
 * He is the rarest Skulkin, only appearing in the two largest and most expensive sets.
 * He is the only minifigure in Ninjago with a moveable mouth.
 * His Skulkin counter-part is Frakjaw.
 * He has four arms, like Lord Garmadon in the 2012 version.
 * Essentially, he is the Skeleton General of Fire.