Distortion is a recurring phenomena in Project Moon media, first introduced in Library Of Ruina before being expanded upon in The Distortion Detective and Limbus Company.

The Distortion… is a phenomenon that arises from deeply personal psychological shock¹. Someone undergoing the Distortion process will hear a warm, alluring voice² unable to be heard by anyone else from (Carmen) and began communicating to be persuaded into giving in to their feeling. If the person choose to embrace these feelings, agreeing with (Carmen)'s discernment of their life, they give "consent" and therefore distort. A distortion can also be caused by being in proximity of a Monolith which can be forcibly interrupted by outside force³.

Distortion causes an individual to change into a distorted/monstrous version of themselves, which is usually permanent (and can eventually result in the individual transforming into an Abnormality), but can be reversed. One may even manifest E.G.O after distorting, or vice versa.
If a person fails to either distort or manifest E.G.O, they may turn into a Peccatulum.

Notable Distortions:

• The Pianist
• Papa Bongy
• The Crying Children

References

1.Intervallo I: Hell's Chicken - Episode 3 Faust's summary on Distortion phenomenon
2.Arknights - EX: Pilgrimage of Compassion - Episode 13 Heathcliff and Meursault's comment on Carmen's voice
3.Intervallo III: Yield My Flesh to Claim Their Bones - Story Dungeon Meursault stopping Aeng-du's distortion