Demons forced to obey Amon, including Marchocias, were given the title of "Fixers". Fixers pledge to follow Amon as loyal subordinates, but their pacts include the right to assassinate their lord should they find a chance. In other words, the Fixers are authorized by their lord to betray.
When the Fixers had just pledged loyalty, they planned together many times to kill Amon. Yet, after few months, the frequency of their traitorous actions dropped drastically. The stronger understand the truth quicker, that their target of assassination is so mighty that they could never match, that the title of archduke earned by Marchocias is merely one of the trophies that Amon had obtained millennia ago.
Time had passed, and Marchocias had ascended as a much powerful demon, known as "Amon's Claw". He had torn thousands and myriads of rebels apart, yet he was still unable to separate Amon's head from the body. I shall be the one to hunt that guy---the Fixers had become aware of each other, fearing that their "brethren" would steal their prey. To surpass Amon, the Fixers had become stronger, not knowing that their actions simply protected Amon.
An archaic castle is protected by the strongest assembly of traitors. In its depths the tremendous demon sits on his throne, resting his cheek on his hand with a grim sneer.