Former Demon Lords are those monsters who held the top spot before the current SuccubusDemon Lord. For all the uncountable ages of the world except the current one, the Demon Lords have been grotesque, bloodthirsty horrors commanding hordes of gibbering, flesh-eating monstrosities.
Little is known about any of the previous Demon Lords, although there is a little information on the one directly preceding the current officeholder. Historical events during the last reign included the defeat of the Pharaohs and the breaking of their desert kingdoms, as well as the war between Poseidon and the Demons of Chaos. Aside from this, information is very sketchy; neither its species nor gender is known. Even its ultimate fate isn't specified, although the usual fate of Demon Lords is to be slain by humanheroes sent by the Chief God, and it seems probable that this is what happened to the former Demon Lord.
"Demon Lord" isn't just a title given to the strongest monster; there is metaphysical baggage that comes with the job. The Demon Lord serves as a sort of nexus for the monsters and their demonic energy; "Fear and respect towards the Demon Lord is imprinted in [monsters'] instinct. All of the monsters are subordinate to the Demon Lord, and they know from birth that the Demon Lord's energy flows within them. In the age of the former Demon Lords, the Demon Lord ruled the monsters through terror and fear using its immense power" . Wandering Scholar relays the monster's account that the former Demon Lord - and all its predecessors - were somehow being secretly manipulated by the Chief God. According to them, when the world was created by the original Chief God, the reason that Demon Lords were included in the grand design was to maintain the yin-yang balance between the number of humans and the number of monsters. When monsters grew too numerous, the Chief God would empower human heroes; but when the humans grew too numerous, the Chief God would manipulate the Demon Lord to lay waste to human civilization. Needless to say, Order dogma strongly refutes this as grotesque heresy, and maintains that the first Demon Lord and the monsters "emerged from the darkness"  contrary to the gods' plan.
When a Demon Lord falls, what usually follows is a chaotic and bloody interregnum where the surviving monsters butcher each other in an effort to secure the title for themselves. Since this is the period of time where human civilization generally builds itself back up, the position of Demon Lord can presumably lie vacant for centuries or millennia. In a departure from the usual course of cyclic history, this isn't what happened after the former Demon Lord's destruction. Instead of civil war, the monsters united behind a succubus to quickly produce a new leader .
However, the hierarchical relationships aren't ones of absolute obedience like they were in the age of the former Demon Lords. As of the present, the hierarchical relationships have become extremely loose. In the age of the former Demon Lords, lower ranked monsters were practically treated like slaves by the higher ranked monsters, but in the present it's entirely formal during times such as when maintaining a dungeon, capturing men as a group, or battling the forces of the Order. Outside of such situations, they mostly have a basically equal relationship.But even so, organizations such as the Demon Lord’s army and Sabbath, etc. do definitely observe a hierarchy, but although the monsters respect and love the higher ranked monsters, the lower ranked monsters don't absolutely obey the higher ranked monsters, and the higher ranked monsters don't restrict the lower ranked monsters.
Especially when it comes to squabbles over human men, rank is meaningless, and they compete equally as individual females for the right to mate first with the male. If lower rank monsters were given orders that interfere with them having sex with men, they would of course express displeasure, and it's also understood that the higher rank monsters should refrain from issuing orders that get in the way of the lower rank monsters having sex with men. In the age of the former Demon Lords, lower rank monsters would often kill higher rank monsters and steal their positions, but as of the present, since the monsters hierarchical relationships are loose, and they lack a lot of interest in things other than sex, such killing no longer occurs at all.
The “Demon Lord” who is considered to be an absolute being by the monsters is an exception to the rule of vague, loose hierarchical relationships. She reigns supreme over the monsters. Fear and respect towards the Demon Lord is imprinted in their instinct. All of the monsters are subordinate to the Demon Lord, and they know from birth that the Demon Lord's energy flows within them.
In the age of the former Demon Lords, the Demon Lord ruled the monsters through terror and fear using its immense power, but, appropriately for a succubus, the current Demon Lord rules the monsters by giving them more pleasure and happiness. The current Demon Lord doesn't bind the monsters through power and laws, but the Demon Lord and the monsters are linked through demonic energy, and the monsters are strongly influenced by her, so it's possible for them to know her will to a certain degree. Formerly even the Demon Lord position was decided by monsters killing each other. New Demon Lords rose to power through rebellions, but the current Demon Lord prefers pleasure, and doesn't like killing, and because of the way she rules, the monsters of the present have no desire whatsoever to rebel.