The Void sickness that overcame monsterkind, a reaction to the mists of the Void leaking from the Glitch. It causes heightened aggression, paranoia, and extreme, insatiable hunger.
The tear in spacetime that Flowey created on accident, through his own ignorance and inability to control his power.
The space between universes, filled with otherworldly creatures. Not meant to be accessed by the living, though powerful monsters can travel through it for a few moments.
Monsters still fall to dust if their souls are overcome and extinguished, but the monsters of this universe have grown clever. They keep their prey alive while they are consumed/harvested, allowing their souls to shatter only when they have successfully gotten everything out of them possible.
The magically talented offspring of monsters and humans. Basically a monster with a human soul.
Two souls destined to be one, linked across both space and time. Once tied together, soulmates will share their longevity, and often perish at the same time. Frisk and Sans were soulmates, before her death.
Frisk’s soulmating gift to Sans. He will never remove it.
Sans’ soulmating gift to Frisk. Aliza found it among her mother’s old belongings.
A gathering of Frisk’s insane ramblings from her time in the Asylum, detailing much of the Underground through maps, sketches, and rambling explanations. Aliza created a compendium of them to follow for her own journey.
The skeleton brothers’ secret weapons, ageless Void beasts discovered by their father that they can summon in times of need. Unable to be brought into the Underground now, due to the sheer amount of corruption they suffer from the brothers’ tainted magic.
The jars that once contained the six human souls, sent Aboveground with Frisk on her journey. Only one survived the long years, and Aliza used it to gather the seven souls she carried with her to the Underground.
A twisted, easily summoned bone attack, studded with an extremely sharp axe blade. Most of Sans’ magic is too painful to use now, and so he defers to this, as it causes minimal pain.
Save stars can only be accessed by the most determined being in the underground. They are bright, concentrated points of powers in time and space that one could use to save a specific point in time.
Most of the Save stars are destroyed by the hunger's corrupted magic, very few save stars are left in the underground in places untouched by the hunger (like the sunken kingdom).