Inexplicable. Summoned merely by mention.
That man accepted the gifts of Lzothozsh, and grew great with power. Over the years he lost himself in the void of power. He became paranoid of his own ability to exist as he was consumed by the black goop that was his form. As a child of Lzothozsh, he adopted the name 'The Corruption'.
A portal to the Corruption was uncovered long ago. One family, the Flavaar's, sought to use it's power and set up their home nearby, claiming the land. For many years the family used the Corruption's gifts until their line dwindled. The most recent of the family is Drakus Flavaar. Another who has accepted the Corruption is Mary Bloodtear.
Then the Corruption grew tired of the isolation brought about from it's form. It shed it's power to four others and made itself mortal once more. Little did it know in this act it had actually (technicaly) impregnated the four women with it's foul spawn.
Two of them escaped with mental sufferance, one was killed (but returned) and the last became the priestess of Lzothozsh.
A new Corruption is formed to serve while Lzothozsh slumbers.