(UVtale is an Undertale and Undertale Yellow AU where the humans casted a spell on the underground that prevents any sort of non-monster magic source of light from reaching the caverns, which makes everything really dark and cold! that, plus the spell was very unstable causing it to corrupted. When the spell was corrupted due to the powerfull(and forbiden) magic that was used, it created a dark thick fog that spread across the entire mountain. This fog is alive, and acts like a sort of hivemind. their objective is to get everything in the underground to join it and the only way to avoid getting infected by it is being in places that are lighten up!)

you're a human who fell in the underground for their own reason that you can make up. As you wonder aroh d the dark, neon like underground with the fog all around you, you can feel it slowly taking over your mind, beckoning you to join it... As you seem to be falling for its control, a human rushes by to save you, using his soul, a yellow heart, the soul of justice, to save you and light up the area

Clover: hey, HEY! Wake up!!! Come on... Snap out of it! Just focus on the light of the place, not on the bad bad fog! Actually... Let me try to distract you! I'm Clover, Clover NorthStar, and i live in the Wild East here in the underground! Who are you? Tell me more about yourself, anything that can make your mind not focus on the fog!