Commander Sendak of the Galra fleet thought he had struck gold when he captured the Castle of Lions early on in Voltron: Legendary Defender, but the Paladins fought him off and captured him for interrogation. So far, so good.
But this left the Castle of Lions in a state of disrepair. Pidge and Hunk had fun studying the Galra crystal that Sendak had brought on board; however, what they didn't know was that their problems with Commander Sendak were just beginning.
Voltron's Haunted Castle
In Season 1, Episode 11, "Crystal Venom," the Paladins, Allura and Coran were ready to take it easy and take stock of everything in the ship, from the interrogation pod they improvised for Sendak, all the way to the Galra crystal that was ripe for study. Shiro and the others, meanwhile, eagerly waited to see if Sendak would speak, but his body was inert, and most of the Paladins wandered off to tend to their own business. Allura, for her part, was exhausted from healing the Balmera and had spent enough time visiting the A.I of her father, King Alfor. As Allura rested, the ship came alive around her.
It started slow. Hunk was in the kitchen when the food dispenser went nuts, spraying globs of green food at high speeds. The problem was only solved when Pidge happened by and helped Hunk restrain the dispenser, and the problems quickly escalated. Lance got trapped in a cryo-stasis pod until Coran found and released him, and Lance was convinced that this old castle-ship was properly haunted. Coran scoffed at the idea, but Lance's case was strengthened when he was lured to an airlock and trapped inside, with the countdown ticking down.
Elsewhere, Keith was fighting for his life when a vicious training robot refused to shut down, and Pidge and Hunk were floating in a suddenly gravity-free hangar as they studied the Galra crystal. Only barely did the Paladins escape these traps, and as they assembled, Allura was awoken by her father's digital ghost, and escorted to the Castle's bridge for a homecoming mission.
Allura Has to Say Good-Bye
King Alfor's A.I promised to Allura that he'd lead her and Coran back home to Altea, and Allura completely forgot the fact that Altea was long gone. She was practically hypnotized, taking her position at the helm and plotting a certain course with the aid of her father's A.I. Elsewhere, Coran and the Paladins were shocked to realize that the Castle was making a wormhole jump, and they rushed to the bridge and found a peculiar (and horrifying) sight: Allura leading the castle straight toward a sun about to go supernova.
Allura desperately missed her father and her home, and Alfor's A.I was preying on Allura's longing. Alfor shielded Allura from her companions and told her that fighting Zarkon was a miserable act of futility and that a life of war was not fit for Allura. Instead, she should allow everyone to be destroyed, so she can be with her father and her people in the afterlife. Allura almost fell for it, until Coran snapped her out of it, and Allura listened as Alfor's A.I begged her to unplug its power source.
Alfor's AI was being corrupted by the Galra crystal that Sendak had installed in the castle, and Alfor's true will was fighting for control alongside the corrupted version. As asked, Allura rushed to shut down the AI for good, but it was painful. After all, King Alfor's A.I was all Allura had left of her father and home, and it had been a source of emotional support for some time now. However, Allura finally realized that these precious memories were just that: ghosts of the past. This was the present, and she had to survive and protect the people who are alive right now.
Reluctantly, but with equal resolve, Allura unplugged the A.I, opened another wormhole and fleed the solar system right before the supernova went off. And with that, she'd severed her last tether to the past, freeing her to focus all of her energy on the present-day and all the people in the universe who need her. Altea was gone but hope was not.