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Temmie's Game Face at the start of the game. Anyone who knows Underswap (or even knows Undertale and saw through their act because they're where Flowey would be) knows that Temmie will turn out to be evil and trying to kill Chara, but that doesn't make them revealing their true colors any less nightmarish. Their face changes into a freaky skull-like face dripping black liquid and with a massive jaw filled with sharp teeth, they suddenly extend their arms inhumanly long to grab Chara, and their paws become giant claws that trap Chara's SOUL in place for the killing blow. Congratulations to the team for actually making a Temmie scary.
"So do me a favor, and... (starts to stand on their hind legs) GIVE... (fully stands up) ME... (sporting a delightfully creepy Slasher Smile) YOUR... (their arms extend, with their paws becoming massive and claw-tipped) SOUL...!" (cackles as they trap Chara's SOUL)
The overworld music in Ruthless, known as "A Special Feeling". Dear God. The longer you listen, the freakier it gets, becoming unbearably tense and creepy, to the point it sounds like it came right out of a horror movie. There's a little giggle in some parts of it, which raises the question of who's giggling?
At some points in the song, you can hear slashing sounds, as well as the sound for damaging a monster.
Once the kill count for an area is exhausted, the next encounter dialogue says "Nothing but silence".
At the City of Old in Ruthless, there's barely any NPCs in the overworld, compared to the plucky little place full of colorful and friendly people in Compassion. Instead, everyone is in hiding and will slam and lock their doors as Chara comes near, hinting they're all watching through the cracks in their door for Chara, hoping they'll see them leave and know it's safe.
The out-of-nowhere and very creepy dialogue-changing sequence at Asgore's house and our player character's mounting confusion and fear over it. We don't know who or what is responsible, and suddenly how much control over their own actions Chara even has is thrown into question.
Going to bed on a Ruthless route has Chara ponder their situation, and when they wake up, one eye opens, showing the same red glow that appears at the beginning of the game, before they suddenly dive out of bed.
Trying to go back to sleep has Chara just go "..."
The Reveal that there are seven soulless Temmies as the main villains of this game, and all of them are as demented, creepy, and hellbent on taking your SOUL as the original Flowey was. And they're not afraid of playing the number advantage either: they circle around Chara, taunt them about how no one will even hear them scream, and close in on them while laughing their heads off, ready to kill Chara where no one will find them or help them, right after Asgore let them go trusting they'd be safe.
During the same scene, an extremely unnerving track plays, which is actually Temmie's Leitmotif, "Fuzzy Friend" slowed down.
Remember how Flowey stalks Frisk in Undertale? There are places you can see Temmie doing the same in the demo, and with seven of them on Chara's tail, it means Chara will be under constant surveillance by a whole group of sadistic killers, watching from the shadows and itching to get them alone to kill them. At least Flowey can keep them at bay (and it's hinted Papyrus will also be watching over them), but if they see an opportunity...
Temmie: We'll leave the human be, for now... But sooner or later, they're bound to fall into our clutches. We both know you won't always be watching over them, "Flowey."
If you backtrack at certain points in the Starlight Isles, you'll catch a glimpse of a shadowy creature, most likely one of the Temmies.
Instead of a funny Breaking the Fourth Wall sequence, the Ruthless version of The Stinger ends on Sans as Crossbones threatening Chara with no music playing, cut to black, and then a slowed-down version of the already unsettling "In My Way" plays as a grim reminder that, hero or not, Sans is still destined to become another victim of Chara in the full release.
Harry and Larry are legitimately terrified of you upon their second encounter on a Ruthless Route. They panic as you move towards them, the background music growing slower with every step you take. Eventually, they escape and run up the mountain, giving you the choice of pursuing them or sparing them, thereby aborting the Ruthless Route. Should you decide to corner them atop the mountain, the terrified duo fights back, thus initiating a miniboss encounter. The problem is, they heal each other whenever their HP reaches 1. The only way to kill them is to attack Larry until he reaches low HP, then push him off the cliff. Should you do this, a despondent Harry falls to his knees in despair, allowing you to kill him.
The pre-battle cutscene. As you corner them on the mountaintop, the duo panics as they realize that their helicopter hats are out of fuel, thereby preventing them from flying away. Furthermore, they left their phone at home, meaning that they can't call for their boss to save them. Throughout the cutscene, you are repeatedly given the choice to turn away, and the scene can only be progressed by choosing to move forward. All while thisDrone of Dread plays in the background.
The music for the encounter is a dark, dissonant number which manages to be in equal parts unsettling and blood-pumping. The 30-second mark introduces frantic violins with deranged laughter and demonic-sounding garbled speech in the background.
This piece of dialogue has some unsettling implications.
Harry: ... Larry, we... we've got to fight the kid. Larry: Dat kid?! B-but… But it... It don't gots a SOUL! [The music stops abruptly.] Chara: My... SOUL? [Chara twitches violently for a moment before engaging the duo in battle.]
The flavour text likens them to a pair of cornered animals.
Interacting with the strength game on a Ruthless Route will have Chara repeatedly hammer away at it until it completely breaks down. Subsequent interactions with the smashed machine will result in Visible Silence from Chara.
Using the camera at the abandoned festival while on a Ruthless or Evacuation Neutral route will result in Chara taking a selfie. Viewing it in the Collectibles shows their glowing red eyes in their Face Framed in Shadow, creating a very unsettling visual.
Going into the attic of the hotel and sleeping while on a Ruthless route has Chara ponder why this feels so familiar. They then lament that the strength they have isn't strong enough for "them". And then they repeatedly ask for them to leave them alone. And then some shadowy creatures almost attack them while they sleep, only for Flowey to suddenly show up.
After beating Crossbones in Ruthless, if you go back to the echo flower stand in the festival area, you hear Temmie's soundbite when interacting with the Echo Flower, with it saying "We're still watching you."
. Dear God. The longer you listen, the freakier it gets, becoming unbearably tense and creepy, to the point it sounds like it came right out of a horror movie. There's a little giggle in some parts of it, which raises the question of who's giggling?