Nightmare Fuel / The Final Timeline

• Flowey may only show up in one chapter of the story, but he manages to act completely unhinged. Cackling like a lunatic, he's overjoyed at the thought that Frisk is giving him her soul so that she can join him in "showing these pathetic monsters the real meaning of this world." It's a relief we never have to see him again... right?
• W.D. Gaster. Everything about W.D. Gaster.
  • To elaborate, W.D. Gaster, The Man Who Speaks In Hands, is a dark, stoic monster who has no problems about hurting innocent people if he believes it will be worth it in the end. He attempts to extract the Determination from Asriel, succeeds in extracting Frisk's Determination, and nearly kills Asgore and Papyrus- his former boss and his son.
  • What's scary about him isn't the things he does; it's how calm he is while doing them. He almost never breaks his facade, even when he's in the middle of a fight. It makes it seem like he's completely in control of any situation he sets foot into.
  • He can create clones of himself.
  • Even before he's fully reconstituted, Gaster absolutely dwarfs everybody when he's in battle. In addition to his cloning ability, he can summon bones, slash away at people with his scythe, and telekinetically fling people around like ragdolls in a hurricane. This is before his powers are at full capacity.
  • He's also nigh impossible to kill, as demonstrated when Sans fires a bone straight through Gaster's heart. What does he do? He doesn't keel over, he doesn't scream in pain, he doesn't even flinch. He just pulls the bone out like a splinter and hurls Sans into a wall with his powers.
  • When he's finally reconstituted, his stats return to their former Mark of the Beast glory, making him twice as strong as before. Don't worry, though! He can finally be hurt, as demonstrated by Sans when he rains lasers upon his form with his Gaster Blasters... until it's immediately made a moot point when Gaster takes control of the Blasters. He would have killed Sans if Papyrus hadn't intervened.
  • He also nearly kills Asgore with one slash of his scythe. It's a good thing Asriel intervened, even though he- surprisingly- merits an entry of his own.
  • When Asriel has Gaster on the ropes, the scientist swan-dives straight into a Villainous Breakdown, pulling an Emperor Palpatine and electrocuting everybody with black lighting bolts from his fingertips. He then makes it clear that, even if it Nothing Personal before, Gaster will banish the group to the Void, doomed to be as helpless as he was as they watch the universe march on without them. If Frisk didn't recieve a Heroic Second Wind, Gaster would have won.
• Chapter 6 sets off all sorts of alarm bells. (Well, besides it being the chapter where W.D Gaster is resurrected) First, it's significantly shorter than the other chapters, only containing a few lines and no Previously on… blurb. Secondly, the description is redacted except for the spoiler warning. Finally, the top comment is written in Wingdings which, when translated, are a little... ominous.
  A BROKEN MIND
  DOES NOT LOSE
  THE ABILITY
  TO THINK
  IT JUST LEARNS
  TO THINK
  DIFFERENTLY
  THAN OTHERS
  ...
  DOESN'T IT?
• Before her Heel–Face Turn, Undyne can be surprisingly scary. From threatening Frisk and telling her that King Asgore is the only thing keeping her from killing the human, to having her monitored by Papyrus even after Frisk was proven innocent. Undyne's scariest moment, however has to be during Frisk's trial. When Toriel suggests allowing Frisk to be exempt from all human-related laws, Undyne completely loses her shit, screaming to Toriel that her time in The Ruins messed with her head. After all, why would she praise a human as a hero when it was humans who were responsible to Asriel's death in the first place?
• At the party celebrating Asriel's return, Undyne checks in with the Royal Guard, who are stationed around the outside of the castle. All of a sudden, Doggo spots a mysterious figure... who disappears before he can get a closer look. Before Undyne can tell him what to do, the line goes dead. She later finds out that the mysterious figure, who is revealed to be Gaster, subdued the ‘’entire Royal Guard’’ within moments of the line going dead. A quick peek into how incredibly powerful the Man Who Speaks in Hands is.
• Rage-Fueled Asriel. Beware the Nice Ones incarnate.
  • To Elaborate: Frisk's Determination has just been extracted, and Gaster's been fully reconstituted. He critically wounds Asgore, but before he can deal the killing blow, he's wrenched off of his feet and thrown into a wall by a vine. That vine turns out to be Asriel's arm. Asriel's flown into some mysterious rage, one which terrifies everybody.
  • All does is glare at Gaster, and the skeleton monster takes a step back in fear. When Gaster, the Mad Scientist who never loses his cool, is scared of something, run away from that thing as fast as you can.
• While the Amalgamates only show up show up for a few moments and are mostly used as a Butt-Monkey in an otherwise tense scene, they take the cake for being incredibly terrifying in the space of a few lines. They're able to scare the crap out of Asriel, breaking his otherwise defiant attitude for a moment. It doesn't affect Gaster in the slightest, of course.