- Mano, a recurring enemy of the Legion and a walking Person of Mass Destruction who has the ability to disintegrate anything he touches with his hand, known as the anti-matter touch. His first use of this power? Destroying his own home planet out of pure spite.
- Validus' true identity. He's actually Garridan Ranzz, the time-displaced and brain-damaged child of Lightning Lad and Saturn Girl kidnapped at his birth and turned into a monster by Darkseid so he can kill his own parents. Time and again he's had to unknowingly attack his own parents, with his own parents unaware they were attacking and regularly imprisoning their own child.
- The Sun-Eater is a star-eater tentacled Eldritch Abomination. It's so big that its viscous tentacles can envelop a sun completely. And it's not an unique being. It's a species.
- The Time-Trapper, a hooded figure which dwells beyond the End of the Time and is a hostile sentient timeline whose physical identity changes constantly.
- In one of his earliest appearances, he entrusted his henchwoman Glorith with an hourglass that, upon physical contact with a person's bare skin, will reverse their age until they devolve into a puddle of primordial ooze. Glorith tests the hourglass on one of the Trapper's barbarian henchmen and she planned to murdered the Legionnaires with it. The only reason it didn't work was because of a nearby fountain spewing a mixture of different chemicals in the air counteracting the process. And later, the Trapper uses the hourglass on Glorith as punishment for failure.
- Positive Man, a Supergirl enemy. An erstwhile mad scientist transmuted into a planet-sized, intangible, translucent, humanoid-looking energy being who loathes all living beings and is able to obliterate worlds by passing through them.
- Lightning Lord's obsession with his sister Ayla is positively creepy. He's been known to lock Light Lass up and torture her in hopes of get her join him after breaking her mind.
- The fourth issue showed us the Mordruverse, a future where the Legion never got together and Mordru the Merciless succeeded in taking over the universe. One page shows Mano demonstrating a device that lowers prisoners into an R.N.A. bath which dissolves them down to nothing and extracts their memories and information. Mano tests the machine on some poor woman named Prudence, who can only scream helplessly as her body's slowly lowered into the bath until there's nothing left of her.
- The first annual showed us the depths of Glorith's pettiness when she found out Ultra Boy had figured out her plans before anyone else did, and secretly undermined everything she did without her knowing. How does she pay him back? She kills Phantom Girl in an explosion right after Jo proposed to her. But it didn't stop there. Many believed Tinya had really survived and was sent to the 20th Century where she became Phase of L.E.G.I.O.N., but when Jo travels to find her it turned out no, it was actually Tinya's cousin, Enya. Glorith didn't just make Jo believe Tinya was dead, she gave him a slight chance that Tinya was alive only for it turn out to be false hope.
- Dirk Morgna's fate. His attempt to redeem himself only gets him shunned by Earth, then he's caught in a nuclear blast that turns into a burnt, skeletal corpse but doesn't kill him. Then his lover Circe puts a bullet in his skull to end his misery before she kills herself. And you'd think it would end there, but then his corpse is revived by Mordru, and then it gets unintentionally hijacked by Wildfire's consciousness.
- After returning from having lost two friends to a third, having their hovercar blown up around them, and nearly drowned by a wall of acid, the missing legionnaires discover various skeletons; including one with Leland Mc Cauley's glasses, and another with the helmet of Amilia Crugg (aka the Khund lady that had been watching over Lori Morning for them).
The future can be a place of horrors!
- Aside from the very idea of someone like Lemnos existing in any century; one, and possibly more, of his fanatics earns a place here by having a transmatter gate deactivated on him vertically while he was attempting to encourage those behind him to ignore the Legion's raid on their secret hideout.
- During the Dominator War, the Dominators capture and torture Cosmic Boy and Triplicate Girl. It's not pretty as one Dominator slowly and partially splits Triplicate Girl while painfully electrocuting her body.
- Princess Projectra's FaceHeel Turn gets solidified when she brutally assaults Phantom Girl and puts her in a coma, then performs Mind Rape on Saturn Girl to keep her from alerting the rest of the Legionnaires. And the worst part is that this volume of the Legion's run ended with Projectra never getting found out.
- Satan Girl. In the Silver Age continuity she was Supergirl's Evil Twin, which is already rather scary per se. In the retroboot she's an other-dimensional goddess of love and death that took over the world in a matter of days.