He is a great demon of antiquity who held a rebellion, which was impressive enough to be described as "legendary" in Dark Zone, a place where betrayal and rebellion are nothing but daily events.
In an era when Dark Zone wasn't as turbulent as it is now, when there are only few well-known rulers---"Frederick" had served the demon lord with the greatest territory of that time. His prowess is outstanding in all aspects, including but not limited to wars and politics, and no other retainer was comparable to him.
He was so loyal, obedient and competent, that the demon lord trusted him totally. And he became the first retainer of the demon lord...on the day he earned the title, he cut the head of the demon lord who trusted him carelessly. Even comrades cannot be trusted, is the rule of Dark Zone---to make everyone forget the rule, he feigned as the best servant.
As such, Frederick gained the greatest share of Dark Zone's territory in one night. He is the evilest traitor who destroyed a mightier being with the sweet trap of "false trust". After defeating the demon lord, the only being stronger than him, Frederick suppressed those who opposed him with force, and slayed them without hesitation.
Whenever he is stained by someone's blood, his lips flickered in lunacy. Such form inflicted fear and earned him the title "Demon Lord of Fresh Blood". However, in historical records of later ages, he is always described as "Rebellious Retainer" instead of "Demon Lord". This is perhaps due to despise, towards a traitor who could betray in a way that could even disgust demons.