CorruptedTale or CT for short, is an AU where the underground is coated with an unusual level of corruption in the air underground. Though harmless in small proportions and to humans, it can cause significant changes to monsters when the corruption is too high or at night for long periods of time, causing their appearances to change drastically and mess with their free will and intellectual minds, turning them into mindless, primal and savage beasts. In Pacifist, the corruption slowly grows stronger over time, so staying for too long isn't the best idea if you're attempting to get out fast. Frisk must make sure to purify any monsters tainted by it and not commit the sin of earning any LOVE earned or they risk being controlled by Gaster and being corrupted from the stagnant air. The levels are as followed:
LV 1: Pure
LV 2 - 4: Innocent
LV 5 - 7: Moderate
LV 8 - 10: Sinner
LV 11 - 15: Mischievous
LV 16 - 18: Demon
LV 19+: Corrupt
The higher the level, the more easily it is for Frisk to become overcome by Gaster's influence. Once Frisk has gained another level above LV 1, she is unable to purify any monsters from the corruption. If Frisk goes Genocide, the corruption only increases faster due to the negativity, causing the monsters to quickly change into primal beings. They are easy to recognize since they have a mark that's in the shape of an eye with an X across it.

A word of warning; the corruption does not lessen or go back to what it previously was upon death or during a RESET. It can only be completely reset on True Resets. Be cautious on your choices!