you are missing some words. The full quote is "The leprosy of unreality disfigured every human creature in attendance upon Monseigneur."
First, some context. Leprosy is a disease that causes horrible disfigurement, usually in the form of missing extremities. Monsigneur is a famer general, a powerful french noble, who has a large amount of money and influence. At the moment of this phrase, the farmer general is throwing a party.
Dickens is comparing the opulent, unreal lives the nobles lead to a case of leprosy because the wealth and power corrupt these nobles who the farmer general has invited to the party.