The title of Demon Master is reserved for the most powerful of demons. The ones that control thousands of lesser demons known as familiars. They are have the power to create their own world within their universe and rule it as they see fit. Until the last book in the series the only Demon Master we know of is Lord Loss, and as powerful as he is it becomes apparent that he is not the most powerful. Demon Masters cannot cross to the human universe via a regular window as they are too powerful, or to big. They have to wait for a tunnel to be opened and for it to become big enough for them to fit. Though a tunnel is opened in the fourth book no demon masters get through before it is closed. Lord Loss is the only demon master who can cross between the human and demon universes whenever he well pleases.