Immutable Hierarchy

Tyrannus, Hierarch of Chains personal army, rampaging through the multiverse ever since the Twilight War - but actually way before it, due to the timeless and retroactive nature of the gods. Today the Hierarchy is one of the strongest threats that the Empire has to face, especially as it is also the one most commonly underestimated.
The Immutable Hierarchy is seen as one of the ‘dumbest’ of the Fallen’s factions. It’s true that its members aren’t thinkers (much less overthinkers) like the members of for example the Shadowlight Covenant, and they lack all the subtleties of the Rapturous Ecstasy. However they make up for it with pure power and the fact that they actually tend to be more intelligent than people think. The result of this wrong stereotype are many, many horrible things happening to people who didn’t deserve it.
Hierarchy is, among other things, a materialized tyranny. Power and control is their goal, and not a method. They seek to conquer, dominate and enslave everything that moves [and a lot of things that don’t, if possible]. Each of their supernatural servant is driven by the same insatiable desire to control and command everything as their most supreme patron, the Hierarch of Chains. If they ever win, the entire multiverse will turn into a place that makes the world of the 1984' appear as a place of total liberty and anarchy.
Its realms - both in the Lost Lands and in the higher levels of the multiverse - are a truly immutable hierarchies, where rebellion is impossible and unthinkable and all toil endlessly to satisfy the demands of those greater than them. Initiative and creativity are seen as a threat and expunged. There is no place for art, no place for freedom of expression, no place for freedom as a whole. There is only tyranny and servitude.
Its supernatural servants are a reflection of that ‘philosophy’. They are powerful, imposing and terrifying. They lack subtlety, but they make up for it with pure strength. What’s more, the fear they spread is supernatural in nature as well and is a mind killer in a literal sense. If one is overtaken by it to a sufficient degree, they are broken by it and become a puppet of whatever being that broke them. It might be irreversible [if the breaking entity was really powerful or if they stayed like that for too long]. Only the strongest can resist that influence - and that's the type of the people who are the most likely to fall and join the Hierarchy willingly.

Structure

The Hierarchy is unsurprisingly a force of a strict hierarchy. Every single high god, god, demigod, archdaemon, daemon, spirit and a mortal that has ever served (or will ever serve) the Hierarchy has a predefined place in it, that cannot be changed. Properly understanding this system from an outsider's perspective is however a troublesome endeavour. After all, letting the mortals know exactly who answers to whom could endanger the Hierarchy's operations in the not yet conquered worlds. As a result, while a large difference in rank is easily discernible, the smaller ones are often a mystery until it is too late.
The ranks are all numbered, and the higher the number, the most dangerous of a creature. Nobody knows what's the highest rank number (whose holder is, presumably, the Tyrannus himself), but the most powerful Immutable Hierarchy entity that has ever visited this world (a lesser god) was of the 9731th level (which is known because said god has announced that with pride... and as a part of some terror tactic).
Besides the absolute servitude that the lower ranked entities offer those of higher ranks as a whole, there are also personal loyalties to be considered. In a way, many of the Hierarchy's operations appear to resemble a twisted form of feudalism, with the societal pyramid continuing on endlessly. With the average moral level of the servants of the Hierarchy, it's even better described as an entire world of truly evil people, with each and every one of them forever enslaved to someone even worse than themselves. All converging like a spider's web towards the Tyrannus himself, who sits in the very middle of it like a massive spider waiting for its prey... while also sending out orders, micromanaging everything through proxies.

Public Agenda

Tyrannus seeks to conquer the multiverse and depose Honour, Exarch of Authority from his current seat as the undisputed ruler of all the worlds and what's beyond them. To achieve this goal - and establish a tyranny of complete micromanagement done by an omniscient and omnipotent deity that's both a massive control freak and both hates and looks down on everything other than himself. The Hierarchy's wars in the Material Worlds are merely a pale shadow of what happens in the inner layers of the multiverse.

History

Before the Twilight War happened, the gods of the Hierarchy were yet to fall. Of course, the time works differently in the multiverse at large, and gods of their caliber are timeless in the essence, existing in every moment of the subjective mortal past, present and future at the same time. From the very moment that the Fall occured, the Hierarchy gods were always fallen. This is just one of the many things that make sense for the omniscient yet appear to be complete paradoxes for the way more simpleminded mortals.
The reason for the fall is obscured to mortals just as well, but in the end it bottles down to the gods being good, and Tyrannus being the best one of them all. Something extradimensional attempted to invade the Reality, probably leading to the repetition of the scenario that led to an end of both the First Empire and the Menorian Federation. The gods have unmade the attackers, like they always do. However what they did not foresee was that in this process they become briefly metaphysically 'connected' to the invaders, and thus subject to their greatest weapon - their corruption.
It almost ended the world right there. The only reason it didn't was because co-ruler of Reality and the highest of the High Gods (they shared that seat with Honour, though) has sacrificed themselves by taking as much of the corruption into themselves as they could, freeing what was supposed to become the modern exarchs from the taint before it overtaken them. Unfortunately, they couldn't save anyone - and thus the Hierarchs were born. Unfortunately even more, they couldn't save themselves - even with their superior nature and power, they were bound to fall - and thus the Tyrannus was born.
Ever since then, there is only war.

Territories

Domains

Lost Empires - When the Lost Lands were created, they swallowed many still-inhabited lands. Some of them remained inhabited to this day, resulting in many petty empires existing beyond the light of the Grand Empire of Karadia and the Exarchs. Those that remained sane and loyal were eventually stabilized and became one more province of the Grand Empire - or were evacuated, if the stabilization was impossible.
Some of them, however, has Fallen. Resulting in their entire population becoming thralls to the Hierarchs. In the case of the Immutable Hierarchy it typically happened rather quickly, with a group [imperial soldiers or someone else] seizing ‘emergency’ power for the duration of emergency... and the autocratic governance changing into a total tyranny as one decision followed another. It wasn't long until the true masterminds behind the whole affair revealed themselves.
Today these countries worship gods of the Hierarchy as thousands of slaves toil endlessly, killed and tortured at a whim of their owners. The supposedly ‘free’ ones do not fare much better, as there is also a hierarchy among them - and woe to those who act out of place. The stronger you are the higher you are - but no challenges against the greater authority are allowed under the watchful gaze of the Great Tyrant.

Dragon Graveyard - A Spiritual World controlled by the Immutable Hierarchy that appears to be located relatively close to this world. As a result, its inhabitants and fauna are sometimes summoned into the fray or to bolster Hierarchy's armies and have become known enough to be properly noted down. In fact, there were even cases of 'loyal' mortals visiting it during the ages, though precious few mortals save for magisters or the Chosen Ones have a fighting chance against what dwells there.
It is described as an almost idyllic land of bountiful nature and beautiful landscaped, dotted by skeleton remains of dragons of various sizes - some of them truly beyond belief. Noone knows what sort of dragons they were or what slew them. But the slaughter had to be incredible.
Like all other realms of the Hierarchy, it is a tyrannical, totalitarian nightmare ruled by the select Damned tyrants (surprisingly often high dragons in life) from their fortresses carved out of the massive dragon bones. The most prominent of local fauna are various types of domesticated (more like dominated) blackscaled dragons and wyverns that are sometimes rode by the local champions of the Hierarchy.

Military

Notable Subarmies

The Broken Legion - Victims of the Hierarchy, often former slaves of their worldly dominions. Alternatively petty bullies who did something bad enough to end in Legion’s grip but not bad enough to attain any sensible position within it.
The Broken are the spiritual aftermath of such people, ranks and ranks of barely cognizant humanoid puppets awaiting their masters' command with unwavering loyalty. They are clad in relatively average gear [although not all of them, there is hierarchy even at the bottom] and used by the forces of the Hierarchy as expendable cannonfodder to weaken their enemies.
All of that doesn’t stop them from being surprisingly dangerous at times, especially when accompanied by their masters on the battlefield. They tend to be a form of a force multiplier for the Hierarchy forces at large, being completely happy (or at least willing to) act as living shields to save their masters from death.

Religion

The High Gods

When Tyrannus fell, he took two more High Gods with him. The first one was Furio, the Hierarch of Vengeance. The second one is Iniuria, the Hierarch of Injustice. Together this three high gods form a core of the Hierarchy's 'religion', further reinforced by a large crowd of gods and demigods that follow them (pretty mindlessly, but that's par for the course when you are the Hierarchy.
Tyrannus is a god of tyranny, slavery and exploitation of others, presented as slightly androgynous in built king in broken crown, his body fully covered by a menacing black armor. Furio is a goddess of vengeance and retribution, often described as being perpetually busy writing down all the grudges and petty hatreds that mortals have against other mortals - unfortunately, her presence in the Legion indicates that she cares little about actual justice, and is more of a personification of someone using their power to bully others that have used to slight them in the past. Iniurio is a god of corruption, bribery and injustice, existing to spread the forms of governments where only brutal strength and readiness to spill blood (or gold) can truly get you anywhere.

The Fall

It is quite easy to fall to the Hierarchy's influence. All people with any degree with power over others risk that. A child bullying others in the school is already painting his soul with the colours of the Immutable Hierarchy. Any unjust usage of power against those with less power does that to you. Of course, one can always decide to step down from this path - such is the power of the free will. However if one doesn't do that, the corruption will spread. Bit by bit, sometimes for decades.
At some point the usage of your power (be it political, physical or magical) in wrong ways will start feeling increasingly good. You are also going to start feeling more and more sympathetic to the goals of the Hierarchy, seeing only the anarchistic rabble around you that has to be put in line. This is typically the last warning before the Fall - if even this won't make you change your behaviour (and perhaps ask for some of the purification rituals of the Imperial Cult to speed up the recovery), you will soon Fall entirely. And begin to serve Tyrannus while striving for any bit of power you can, hoping to improve your standing in the Immutable Hierarchy - after all, your level in it is predefined, but you are yet to reach it, meaning that the more power you gather the higher you were destined to be.
At this point recovery is almost impossible. The situation is a bit similar although tends to be a bit harder for the inhabitants of the petty empires of the Hierarchy in the Lost Lands, since those are born into servitude to the Hierarchy. In a way, the 'corruption' and 'falling' part works in reverse there - some people dissatisfied with what they see around them develop a growing supernatural influence of the Exarchs, and will eventually 'fall in reverse', fleeing from their countries to join lands that are a bit more to their taste.