Sendak: Do you really think, a monster like you could be a paladin of Voltron?!
Sendak's Galra crystal turns the Castle of Lions against its crew by corrupting its technology, including the AI of Allura's father, Alfor.
- Adult Fear: You hear all the time about strangers claiming to be a friend of your parents... this time the bad guy can just impersonate the parent.
- Arbitrary Skepticism: Coran says that a lot of "supernatural" phenomena can be explained away by using science. In this case, the castle is "haunted" because the Galra crystal and possibly Sendak have corrupted King Alfor's AI.
- Broken Tears: Allura is heartbroken as she runs past the memories of her and her father, but ultimately goes through with destroying his AI to save the others.
- Break Them by Talking: Sendak's consciousness does this to Shiro, who accidentally awakens Sendak out of the induced sleep and then shoots him out an airlock.
- Cant Get Away With Nothing: Shiro, the conscientious Team Voltron leader, wants to create an AI reproduction of Sendak to interrogate it without waking him up. This is questionable ethical behavior, as Coran notes. Sendak manages to use the AI to deliver a Hannibal Lecture, and Shiro sends him out of the airlock.
- Death by Genre Savviness: The team doesn't want to wake Sendak for fear of him causing trouble in the castle, and they hope that by studying the Galra crystal they can reverse-engineer the advanced Galra tech. The Galra crystal ends up corrupting the castle's AI, and Sendak manages to escape thanks to his AI delivering a Hannibal Lecture to Shiro.
- Everything Trying to Kill You: The technology in the castle aims to harm Pidge, Hunk, Keith, and Lance. Pidge and Hunk get the better end of the deal by having to deal with a kitchen that won't stop spewing food and being stuck in a zero-gravity room. Keith's training gladiator tries to kill him, and Lance is locked in a cryopod and then later is nearly Thrown Out the Airlock.
- Explain, Explain... Oh, Crap!: Coran tries to reassure Lance that the castle wasn't haunted, but after he realizes his words don't help his argument, he brushes it off and declares there's nothing wrong with the castle.
- Foreshadowing: The way Allura destroys her father's memory core via Bear Hug foreshadows the reveal that she and maybe all Alteans are far, far stronger than they look.
- Glamour Failure: The holograms cannot replicate scent. This is how Allura realizes that Altea is not restored and she's not on the planet.
- Hannibal Lecture: Sendak's AI delivers one as Shiro interrogates it about Zarkon.
- Heroic Sacrifice: For Allura and Alfor, Alfor's AI is corrupted but he is still able to tell Allura to destroy it. Which means Allura has to listen to her father basically fight for his life while also begging her to kill him. Destroying the AI destroyed Allura's last connection to her old life.
- Hypnotize the Princess: The AI wakes up Allura and puts her in a hologram where she's navigating the ship back to Altea, and puts her in a field of her native flowers. Coran manages to break the spell by asking Allura to sniff the flowers.
- Last Words: Alfor's final words were a tender farewell to his daughter.
- No Sympathy: Subverted. When the team learns that Shiro accidentally freed Sendak, they're more concerned for his well-being than the fact that he freed their prisoner. He immediately then takes charge to find Allura and to figure out why the ship is making a wormhole jump.
- Reality Ensues: While Pidge and Hunk are stuck in zero gravity, Hunk tries to swim to Pidge. Unlike some cartoons, zero gravity isn't the same as floating in water, so Hunk is just "swimming" in place.
- Red Eyes, Take Warning: The gladiator Keith fights usually has a blue eye, but after the ship becomes even more corrupted it turns red and attempts to kill Keith.
- Screams Like a Little Girl: After Coran leaves Lance alone, the cyropod behind him all of a sudden goes back into the floor and Lance lets out a high pitched scream while fleeing the scene.
- Spot the Thread: When Allura believes she's on Altea and Alfor's corrupted AI tells her that these are the flowers she remembers, Coran asks her to sniff the flowers. When she does, it breaks the hologram.
- Thrown Out the Airlock: The ultimate fate of the training gladiator who attacked Keith.
- The Virus: The Galra crystal causes the castle to go haywire.
- We Can Rule Together: During his Hannibal Lecture, Sendak's AI tells Shiro that fighting is useless and Shiro should join Zarkon instead.
- You Can't Go Home Again: Alfor's AI tries to convince Allura that they can return to Altea, that it survived Zarkon's destruction, in an attempt to send the ship into a dying star. Then to save everyone on the ship, Allura has to destroy her father's AI, and all his memories of Altea.
- Your Worst Nightmare: Hunk declares that the food attacking him was the stuff of nightmares.