An evil Human Resources Director.
A roster is a list of names of people involved with an organization of some kind. A master of the roster would be the head or Director of Human Resources. A person is inherantly evil is consdered to be a monster.
A rostermonster is thus the evil Human Resources Director.
Employee: I'd like to request some time off to attend to personal matters.
Rostermonster: We do not allow time off for any reason whatsoever. You must report to work on time, every day and work your entire scheduled shift. Noncomplaince with company policy will result in termination of employment.