Lord Garmadon is the former antagonist of the LEGO theme, Ninjago, introduced as the main antagonist of the series before being developed into an anti–hero and eventually fully redeemed, becoming known as Sensei Garmadon. He is the son of the First Spinjitzu Master and the older brother of Sensei Wu, who turned evil after being bit by The Great Devourer and tried to take over the world of Ninjago, but was banished to the underworld by Wu. He took his signature helmet from Samukai, and became the ruler of the underworld. He returned in command of an army of skeleton people, including the actual ruler of the underworld, Samukai. Garmadon's goal is to possess the four golden weapons of Ninjago.
In the Ninjago movie, Garmadon is betrayed by Samukai, but the skeleton is destroyed when he tries to possess the four weapons, and a portal to another dimension is opened, which Garmadon intentionally lets himself to be sucked into so he can become strong enough to wield the four weapons from inside. In the 2012 series, he returns from the void, now with four arms and with a new group of snake people.
Despite being evil, he loves his son and does not wish for Lloyd to become him, because Lloyd should find his own destiny because he never got to make the choice to become evil, hinting that the Great Devourer's venom couldn't corrupt his heart completely. He also has shown hatred for the Great Devourer. He formed a truce with Sensei Wu and the ninjas to save his son. Once Lloyd is revealed to be the Green Ninja, both Lloyd and Garmadon are devastated knowing they would face each other in battle. However, he tells Lloyd that even though they're now on opposite sides, he's still proud of him and used the skeleton army to help save the ninjas. In the season finale, he used the four golden weapons to destroy the Great Devourer and got his revenge on the giant snake, and then he ran off with the four weapons. Lloyd accepts he will have to face his father in the future and get his training from not only Sensei Wu but the four ninjas as well.
In Season Three, Garmadon will return, this time intent on destroying the ninja, to prevent his son from reaching his true potential. Allying himself with the Serpentine, Garmadon will create "The Dark Bounty" from pieces of the broken Destiny's Bounty, and travel to the birthplace of the Golden Weapons, uniting the weapons into a stronger one.
Back in Ninjago, Skales leads the Serpentine in his plan to destroy Ninjago City, but they stumble upon a tomb filled with Stone Warriors. The Stone Warriors lock the Serpentine up, then leave to find their master, he who wears the helmet of shadows. The Overlord leads Garmadon to the Helmet of Shadows, and Garmadon takes control of the Stone Army. With guidance from The Overlord, Garmadon instructs his army to build the ultimate weapon, consisting of dark matter. The Weapon is designed to transform whatever it targets into Garmadon's evil image. Using the dark island as his base, Garmadon is seemingly unstoppable. Once the ninja arrive, a war on the island breaks out. Garmadon realizes he will have to fight his son soon, but he doesn't want to.
At one point, the stone army captured Misako, Garmadon's wife, whom Garmadon has not seen since before he was cast into the Underworld. She tells him that Lloyd does not want to fight, which both comforts and scares Garmadon. Garmadon is ashamed of his evil ways, but still invites her to rule Ninjago by his side. Misako lies and tells him that she accepts, but she steals the helmet of shadows from him, leaving him angry and hurt.
When the ultimate weapon, Garmatron is complete, Lord Garmadon leads his troops to the edge of the island, and aims the weapon at Ninjago. Lloyd, Sensei Wu, and Misako arrive and tell him to stop. Garmadon tells them that fate has turned his son, wife, and brother against him, and turning them evil is the only way for his family to be together.
Sensei Wu tells him that the Overlord is using him, and that if he fires the weapon, the Overlord will be released. The Overlord insists that Garmadon ignore him and fire the weapon anyway, so Garmadon decides the Overlord is right, and fires the weapon at Jamanakai village. The balance is then shifted, and the Overlord is freed. The Overlord posses Lord Garmadon's body, and he transforms into an evil, scaly figure with claws, sharp teeth, a tail, and a head resembling a dinosaur.
The heroes realize the "Dark Lord" in the green ninja prophecy was not talking about Garmadon, but about the Overlord. Lloyd begins his fight with the Overlord, trying to tell his father to regain control over his body. Garmadon fights back, telling the Overlord that he will not hurt his son, but the Overlord wins, and nearly kills Lloyd. Still possessing Garmadon, the Overlord, the stone army, and evil Nya teleport to Ninjago City, leaving Lloyd on the island with a broken leg.
Later, the Overlord transforms Garmadon's body into his "True Form". The "True form" resembles a gigantic black dragon. When the ninja arrive in Ninjago City, everyone (Except for Dareth) has been transformed into Garmadon's image. The Helmet of Shadows falls off of Garmadon's head when he turns into a dragon, because it is far too small. Dareth finds the helmet, and takes control of the stone army.
Lloyd battles the Overlord, and wins, transforming everything in Garmadon's image back to good. Lloyd is upset because his father's body is missing. But they find him shortly after that, only he is not the Garmadon they once knew. Garmadon is now transformed to look like his original self (Before cast into the underworld) only much older. Every ounce of evil was drained out of Garmadon, making him good again. His family runs to him, and he is finally reunited with the people he loves.