As a Moments page, all spoilers are off.
The webcomic:
- The whole story. Nonstop. Taking place seven years after the events of Undertale, the entire Underground has gone to absolute hell from there, with the CORE dying and Undyne, the new Queen, being unable to stop it. Consequentially, the monsters start starving to death because their magic food is fading away from the core's death. The monster children start dying first and the adult monsters start losing their minds, killing and/or eating each other. In the case of Snowdin, however, the residents have been eating any human children they come across to fight off starvation and to spite Undyne, who just wants the SOULS to be harvested - this happens after a altercation between Sans and Undyne, which there are no known details of other than the result being part of Sans' skull getting smashed open.
- Grillby got it the worst - after the CORE died, the magic that kept his body from burning itself started fading, leading to his soul being in perpetual agony as he feels himself burning alive constantly, but cannot die. In one particular chilling flashback, right when everything starts going to hell, he suddenly grabs Sans out of nowhere after a long night in his bar and starts pleading for him to help him, his face starting to melt while he screams his head off in agony. And he's been living like this for 8 years.
- The story's main protagonist, the 11-year-old Aliza, has a SOUL composed of fear, although it's implied she has a little bit of determination. This means that instead of being able to manually save, her soul autosaves right before she dies, so not only will she remember how painful or horrifying the death was, but she will have to re-experience the terror right before she died. She also seems to have scars from her previous horrible deaths.
- A flashback reveals just what happened between Sans and Undyne: Knowing how desperate the latter had become, Alphys convinced her to try extracting Sans' eye to harvest its magic. It seemed to work at first... but then Sans somehow comes back and not only destroys the CORE, but personally lobotomizes Alphys just to spite Undyne, making it practically impossible for the Underground to survive without killing humans.
- Prior to Sans destroying the CORE, he witnesses a monster teenager (the purple high-schooler from Hotland) turn to dust right in front of him, and we get a horrifyingly detailed look at their disintegrating face before they die of starvation. Seeing this is what convinces him to propose his plan to repair the CORE, and unfortunately what leads to things going downhill.
The game:
- Flowey not only establishing the world they're in, but revealing himself as the Only Sane Man, is shocking to anyone who has any surface-level knowledge about Undertale. Then, after he promises to help you out, he's killed off. Your only hope is now dead.
- Many moments from Undertale have become this with the added context.
- Toriel saving you is turned from a rescue from a sadistic villain into your new partner being killed off by the very thing he warned you about.
- During the spike puzzle Aliza has an aside to herself, wondering about other kids who met their fate to the spikes, then grabs Toriel's hand harder.
- The scene where Toriel kills the thing hiding in the leaves. After players are almost lulled into a false sense of security, Her presence becoming comforting, they see something shifting in the leaves, and immediately Toriel raises her hands and kills it without a second thought. She tries to justify herself, but that sight of the dust covering the ground is likely to stick with them for a while.
- When you read the book about snails, instead of giving you light-hearted facts like it would in Undertale, it tells you about the snail's radula, a tongue with thousands of microscopic teeth that rips up the food it eats.
