Corruption (also referred to as "Kegare" (uncleanness, impurity)) is a lore term and gameplay mechanic in Mary Skelter: Nightmares and Mary Skelter 2.
Lore[]
"Corruption" is a term referring to contamination that builds up in Blood Maidens as they fight Marchens and get exposed to their blood, or if they consistently suffer a great deal of stress. If this Corruption builds up, there is a risk of the Blood Maiden entering Blood Skelter, a berserker variant of Massacre Mode that trades the Blood Maiden's sanity and self-control for power. Blood Maidens with a high concentration of Corruption have their eyes turn pink even without exposure to Marchen blood, and their speech slowly spirals into negativity before they enter Blood Skelter.
Corruption can be lowered if the Blood Maiden has a restful sleep, and there do exist drugs that mitigate its effects, but before Jack's blood was discovered, there were no means of handling Corruption that Blood Maidens could bring with them on missions.
Physical contact with Jack's blood is capable of purging Corruption. This is later conceptualized as a "weapon" in the form of the Mary Gun.
Gameplay[]
Corruption is indicated by the brightness of a Blood Maiden's Blood meter. The stains will gradually become darker (from bright pink to almost black-purple) as they take massive damage or even get KO'd. If the conditions to enter Massacre are fulfilled with high Corruption, there is a significant chance that the Maiden enters Blood Skelter instead. In Blood Skelter, the Maiden acts erratically, attacking friend and foe alike as their HP rapidly plummets.
Corruption can be removed using Jack's Purge command in battle, or by returning to the (New) Liberated District.
The Rescue Center features a "Purge Corruption" mini-game, where by splattering Blood Maidens with Jack's blood and cleaning it off, Corruption is removed. This increases the Blood Maiden's stats (most notably HP and SP) until they are KO'd