Krux, also known as Dr. Sander Saunders, is one of the Time Twins and the older brother of Acronix.
The older version of Krux comes with a double-sided head; one side depicts his Dr. Saunders persona, while the other depicts his Krux persona. He has a unique printed torso and legs, similar to those of Acronix but with an hourglass decoration printed on his left side. He also has a printed hood piece and a green half-cape, both of which he shares with Acronix.
Young Krux has a dark gray and translucent red helmet piece that he shares with Slackjaw and Vermin; the same mold was used for a dark gray and orange piece worn by Commander Blunck. His torso and legs are unique, though similar to those of his older self, and his head is printed with a younger version of the Krux face from his older incarnation.
Krux is a patient schemer, having spent forty years orchestrating his plan to take revenge for his defeat and that of his brother Acronix. He also despises the various technological innovations that have taken place over that time, preferring the simpler times of the past. 
40 years prior to the events of Hands of Time, Krux and his brother Acronix were the Elemental Masters of Time during the era of the Serpentine War. While Acronix could go forward in time and slow down time, Krux could go back in time and pause time completely. He used these powers to great advantage; going back in time seconds before an opponent could make a move and turning the battle in his favor.
After the Serpentine were defeated by the Elemental Alliance, Krux and Acronix, knowing that they were more powerful than the other Elemental Masters, decided to betray the alliance and use their powers to control all of Ninjago. The other Elemental Masters fought against Krux and Acronix, but the two proved too powerful. With the Time Blades forged by Ray and Maya, Wu and Garmadon fought a lengthy battle against the Time Twins at the Monastery of Spinjitzu. Eventually, Wu and Garmadon used the Time Blades to absorb Krux and Acronix's elemental powers, rendering them normal men.
Knowing that the power to control time was dangerous, Wu and Garmadon used the Time Blades to open a vortex that would leave the Time Blades lost forever. However, Krux and Acronix decided to go through the vortex themselves, which left the two "lost to time."
While Acronix would be lost in the time vortex for 40 years, Krux was somehow immediately teleported back to Ninjago, along with the Reversal Time Blade. Krux then took on the false identity of Dr. Sander Saunders and used eggs from the Great Devourer to breed an army. Krux's army would need weapons and armor, and he knew exactly who could make them. Under the disguise of Dr. Saunders, Krux took his time to become a trusted friend of Ray and Maya. One day, Krux revealed his true identity to the Masters of Fire and Water, and threatened to harm Kai and Nya should they refuse to do his bidding.
The epilogue of The Dark Island Trilogy features Krux alongside his brother Acronix in a vision experienced by Sensei Wu, who describes the pair as "the twins I thought lost to time."
As Dr. Saunders, he friendly greeted the Ninja when they arrived at the Ninjago Museum of History and gave them a tour of the new Hall of Villainy exhibit. Later that night, Saunders was present at the Day of the Departed concert with Dareth. After Dareth defeated the resurrected Kozu and his Stone Warriors, Saunders expressed extreme displeasure at the damange to his museum caused by the conflict.
When the Ninja were called to the museum to help clean up the damage from the night before, Saunders became hysterical when he realized that his thermostat had been destroyed. When the Ninja questioned Saunders about why his thermostat was so important, he simply claimed that the museum needed to be kept at a certain temperature to preserve the exhibits.
After the Ninja took back the BorgWatch prototype from Pythor, they used it to repair the original thermostat. Saunders was happy that his thermostat was working again, but expressed his dislike for modern technology. Zane then noticed an ancient-looking watch that Saunders kept in the museum, and Saunders said that he used the watch to tell time for many years. After the Ninja left, Saunders listened to the humming sound coming from the museum basement.
As the Ninja were cleaning up the Ninjago Museum of History, Krux (disguised as Dr. Saunders) presented the Ninja with refreshments. When the Ninja questioned him about a painting showing Wu and Garmadon in their younger years fighting two mysterious warriors at the Monastery of Spinjitzu, Saunders explained that the two warriors were the Hands of Time, but that they were merely a myth.
Later, Saunders was confronted by Acronix. Saunders took off his mustache and hugged his brother, revealing that he was actually Krux in disguise.
At his lair in the museum, Krux was extremely annoyed by his brother's growing fondness for the modern inventions that he never got to experience, such as the BorgWatch and the BorgPad, and Krux claimed that he despised modern life. Krux showed Acronix a helmet and said that he would return Ninjago to its "pre-modern glory"; revealing the Vermillion eggs.
Later, Krux unleashed a Vermillion warrior at the big unveiling of the BorgWatch; commanding it to destroy the Borg Store and to kidnap Cyrus Borg himself. When the Ninja arrived, Krux unleashed two more Vermillion warriors. While the Ninja fought off the Vermillion at the Borg Store, Zane and Borg fled the scene, only to be confronted by Krux in the disguise of Dr. Saunders. With Acronix by his side, the two kidnapped Borg and took him back to the museum.
Back at the museum, Acronix expressed his displeasure that they had lost most of their armor for the Vermillion in the battle against the Ninja. However, Krux assured his brother that he already had his "best blacksmiths" working on more armor. Krux then revealed that he had released hundreds of Vermillion eggs into the swamps of Ninjago through the sewer beneath the city.
After locking Cyrus Borg in a Serpentine coffin, Acronix questioned Krux as to how they planned to control the growing force of Vermillion that they needed for "phase two" of their plan. Reminding his brother that he had 40 years to devise the plan, Krux then introduced Commander Raggmunk, Commander Blunck, and General Machia; three Vermillion selected from the genetically superior of Serpentine who were thus able to control the other Vermillion warriors telepathically.
When Krux was informed by Raggmunk that Kai had entered the museum, he allowed Acronix to monitor the Ninja through a telescope that he had built into a snake statue. Noticing that Kai had taken a Vermillion helmet, Acronix feared that the Ninja of Fire was onto their plan and insisted that he neutralize him. Knowing that it would draw too much attention, Krux declared that he would attend to the matter himself and ordered the Vermillion commanders to commence phase two of the plan.
Disguised as Dr. Saunders, Krux confronted Kai. When Kai asked if he recognized the Vermillion helmet, Saunders said that it resembled a post-Serpentine War samurai helmet, but was likely a knockoff. Kai explained that he obtained the helmet during the battle against the Vermillion that attacked the Borg Store and asked if Saunders had seen it on TV. This prompted Saunders to explain that he despised TV, reverting to his normal voice of Krux for a brief moment. Kai eventually realized that the Vermillion helmet was identical to the helmets worn by Krux and Acronix in the painting. When Saunders tried to convince Kai that the Time Twins were still a myth, his fake mustache ended up tangled in a snake statue and fell off; revealing his true identity.
A lengthy battle between Krux and Kai ensured, during which Kai discovered that Krux had kidnapped Cyrus Borg and had likely taken Zane as well. As the battle continued, Krux claimed that Kai and Nya's parents were actually traitors who fought with the Time Twins. Due to Kai's anger, Krux briefly gained the upper hand when Acronix revealed himself and prepared to finish off the Ninja of Fire with the Forward Time Blade. However, Nya arrived and helped Kai in the continuing battle. Eventually, Krux was kicked into the same Serpentine coffin that Borg had previously been locked in.
Krux and Acronix, along with the Vermillion army, appeared in Wu's dream.
After the Vermillion hoarded all of the metal they could find in Ninjago City, Krux and Acronix led them through the sewers to their secret lair. When the Time Twins found themselves lost, Acronix gloated that his BorgWatch had built-in GPS. However, the BorgWatch was unable to function because there was no service, and Krux then gloated that this was one of many reasons why he refused to "get with the times". Krux then decided to follow his map, only to be led to a dead end.
Krux and Acronix put Cyrus Borg to work building a machine that employs the power of the Time Blades. When the Slow Down Time Blade arrives, they quickly redirect their forces to seize it. However, Raggmunk and Blunck lose it to the Ninja, but the Time Twins and Machia are confident that they can recover it.
After berating Blunck and Raggmunk for their disobedience and failure, Acronix and Krux lead their forces in an attack on the Temple of Airjitzu where the Ninja reside and keep the Time Blade they have recovered. Eventually the pair engage the Ninja in battle directly, only to lose the Fast Forward Blade. However, Machia is able to reclaim both blades from the overconfident Ninja, and the villains escape with them and a captured Wu.
On their way back to their swamp lair, Krux and his brother promote Machia over Blunck and Raggmunk.
Alerted to the imminent arrival of the Pause Time Blade, Acronix and Krux depart to retrieve it with Machia and Borg in tow. Ray and Maya, upon being found by Kai and Nya, reveal that Krux returned to Ninjago much earlier than his brother and gained their trust while under the identity of Dr. Saunders. He then blackmailed them into helping him in his plans for revenge, threatening harm to Nya and Kai if they refused to comply. Unfortunately, the Time Twins return to their camp and break in on the family reunion, and then force Kai and Nya to help them recover the last Time Blade.
Krux and Acronix take Ray, Maya, Kai, Nya, and Wu to the Boiling Sea, and send Kai and Nya into the hostile environment to recover the Reversal Time Blade. Upon their successful return, Krux double crosses them, having never intended to allow them to help Wu with the blade; after the heroes are frozen with the Pause Blade, the Time Twins and Machia leave them to plummet into the water. They then return to their camp and, after thwarting Samurai X and Zane's attempts to destroy the Iron Doom, board the craft with their army.
Using their new vehicle, the Time Twins land with their Vermillion in Ninjago's past, shortly after the departure of their own past selves, and attack the Elemental Masters of that era. Kai and Nya, who have followed them with Wu in tow, manage to help the past Elemental Masters turn the tide, only for Krux and Acronix to seize direct control of the Vermillion. Using the hordes of snakes to animate the Iron Doom, they overwhelm the Elemental Masters, but are then attacked by the Fusion Dragon. At Acronix's suggestion, they then attempt to travel into the distant future so as to be rid of their adversaries forever. However Kai, Nya, and a healed Wu are able to board the Iron Doom and face them, and the Iron Doom is sent tumbling out of control after Wu removes the Reversal Blade. Kai and Nya are able to return to the present, while Wu, the Time Twins, and the Vermillion remain aboard the Iron Doom as it plummets through time.
- On the box of 70626 Dawn of Iron Doom, Krux and Acronix's names are switched.
- Krux's role in the disappearance of Ray and Maya, and Master Chen's apparent awareness of it, seems to imply that the two may have worked together.