Crux is the second son of the First Spinjitzu Master, Garmadon's twin brother, Wu's older brother and Lloyd's uncle. When he and Garmadon were young the two brothers were best friends, but on the day Wu was born, a dark storm struck Crux and he disappeared for a long time.
Crux became the guardian of the Dark Heaven, serving under Iscara, a master of dark arts. He was one of the many servants of Iscara and became responsible for bringing people into the underground prison.
Years later, Crux was assigned to bring the Ninja and Sensei Wu to the Dark Heaven so Iscara could take their power for the Death Lord. However, when he realised Wu was his brother, he turned against his master to free the Ninja from the Dark Heaven. He sacrificed himself to save them, and ended up being blasted by the Ray of Life, which purified him and made him human again. After that he became something of an ally to the Ninja in preventing the rise of the Death Lord.
Crux was the second born son of the First Spinjitzu Master after Garmadon. He was extremely clever as a young boy and often outwitted his father. On the day Wu was born, a strange pattern appeared in the clouds and the Golden Weapons started to react, causing a bolt of lightning to hit him turning his skin black (like Garmadon when he was evil). Ashamed, to hide his appearance Crux left and went on his own journey which eventually led him to the Dark Heaven.
The Curse of Iscara
Iscara orders Crux to bring the Ninja to the Dark Heaven. He spots them at the Ninjago City market and, after defeating them one by one with his black spinjitzu, takes them to Iscara's underground fortress. Lloyd, Misako and Shawn then go and search for them.
The Prison of Death
When Cole tries to escape, Crux stops him in his tracks. He takes the Ninja of Earth to Iscara for punishment. He is also with Iscara at the end of the episode where she reveals her master plan.
The Council of Elders
While the Ninja are waiting in their cell, Crux arrives with Cole and throws him back in with them. Cole tries to break out using his ghost powers but realises the bars are made of deepstone. Jay also tries to shock the bars, but is unsuccessful and Crux laughs at their weak effort. Later, Crux attends the Council of Elders with the other elders like Risto and Black Samurai to discuss the whereabouts of seven mystical artefacts. Meanwhile Lloyd makes his move and tries to free the Ninja, but is captured by a guard.
Shawn tries to infiltrate the ranks of Iscara's demon army. He discovers their plans and reports back to the Ninja about his findings. He is nearly spotted by Crux, but narrowly escapes.
The Final Reunion
Lloyd finds a secret passageway which leads to the cell of Garmadon and the First Spinjitzu Master. Garmadon tells Lloyd all about his history with Crux and how he supposedly died. Crux later appears with his twin brother, realising what he has done and vows to make things right.
Queen of Darkness
Garmadon is told he has to fight to break free from the Dark Heaven. When it looks like all hope is lost Crux appears and fires his guns at some rocks above Iscara, distracting her. She uses dark magic to fight him and this allows Garmadon to get out. A chaotic battle breaks out with Iscara ending up firing her creation, the Ray of Life, at Crux, making him alive again. The ceiling caves in on Iscara, killing her. Crux, Wu, Garmadon, Misako, Shawn and the Ninja leave using the four portal keys. They then fight the villains which had meanwhile been let loose on the street (due to the absence of the Ninja). However, the shadowy voice of the Death Lord warns them that the battle is far from over.