The Corruption is one of the Entities. It is linked closely to our feelings of disgust, of feeling revulsion and fear of corruption, disease, filth, and manifests as mould, bugs, rot, decay, infection. The feeling of your skin crawling. The Hive and the Flesh Hive are either alternate names for it or sub-fears.
Jane Prentiss was an acolyte of the Corruption.
John Amherst seems to be connected to the Corruption.