Pentagram

Pentagram is the ultimate evil that the empire faces, and a pure manifestation of everything that's wrong with the universe. It was the Pentagram that was responsible for corrupting the Hexmachina into homicidal range, that started the Twilight War and resulted in the reality itself breaking into pieces. It is the Pentagram that is even today attempting to finish the job and finally destroy the universe - or, which is arguably even worse, only change it to its own liking. It is also the only group of the Fallen which is so terrifying that it can make someone else cooperate with the Rampant Bloom, a rare achievement. Even the Umbral Bond might sometimes cooperate with someone against it.
While the Breathless Tide and Rampant Bloom seek to radically change the Reality, the Pentagram goes far beyond their level of change. If Pentagram ever achieves victory, it will reduce the entire Reality into a formless chaos were all words known to mortals will stop meaning anything (or even having any degree of sense), with said chaos eventually used to assemble Reality so radically different that merely attempting to see the resulting state of existence would drive anyone living in the current world irreversibly insane.
As a result, EVERYONE hates and opposes the Pentagram. There's no exception to that rule. Fallen, non-Fallen factions hostile to the Empire, and the Grand Empire itself. Because if Pentagram wins, everyone else loses. No exception. There can be no negotiation with it, because it only seeks to destroy and will not deign to speak with anyone, save for those that were already subverted to its will. Unfortunately, this is also a faction with perhaps the most significant presence in the Material World. It's less of a standard Lost Lands brawls and skirmishes (since that place is actually lacking in Pentagram presence - it's there but it's overhunted by everyone else) and more of a regular invasion.
Worst of all, the gods of the Pentagram also have a plan that - if successful - will tilt the balance in their favour even more. The exarchs in their domains might resist the aftermath of it, but there is no way that they will be a lot left from the material worlds (at least in their current form) if that occurs.

Structure

The organization structure of the Pentagram is largely impossible to be discerned by mortals, at least above certain level. In the Prime World, things are a bit simpler - there are four main invasion forces of the Pentagram (five if you count the remnants of the Hexmachina' forces in the Network, with their actual field commanders unknown (and perhaps not-existing - Pentagram is complicated, after all). Each includes fallen petty civilizations (with a greatly varied organizational structures) and numerous daemons that are typically following much greater entities of their own god.
When you do get into the level of demigods and above, things stop making any degree of sense. They rarely are seen cooperating, so typically every major incursion has a single overlord. The actual details of relationship between entities of this level and above are typically only mentioned in some eldritch tomes of lore, almost always intentionally contradictory and sometimes written in a way that threatens the sanity of the reader. In the end majority of the Pentagram's opponents are happy just knowing that banishment or killing of an overlord of the regional operation of the Pentagram is enough to defeat the mind behind the cult or invasion, leaving the rest in the disarray that's typically enough to turn the tide.

Territories

Each of the high gods (save for Odius) of the Pentagram has established a domain in the Reality - and they are in the stable part of them, not in the Lost lands. Those domains operate on a completely different set of rules that one would expect from such a place, which greatly explains the Grand Empire's (and honestly everyone else's) problems with finding a proper way to deal with them.
Abyss is the worldly domain of Poena, the Hierarch of Pain established within the depths of the Western Ocean. It's creation is believed to have sparked the Synkardian War, with the synkardians being previously unknown sapient species living in the depths of the ocean that has fell victim to Poena's depravity and was inspired into the regular land invasion on the northwestern Grand Empire. It is also the realm that is counted among the ones where the local resistance is the fiercest, although it's not the Empire that's responsible for it.
Instead, the Abyss is engaged in centuries long genocidal war against the Fae of the Western Ocean and their numerous progeny (plus whatever remains of the extremely alien underwater sapients). This is a war that happens under the waters and thus remains largely unknown to the Grand Empire, whose sailors only occasionally end up seeing any proof that it actually occurs (typically taking form of either kilometers long carcass of something sanity-rending floating in the water, sudden storms erupting when the local highest ranked fae goes furious at the casualties or is influenced temporarily by the insanity of the abyss, or the fae that they had to deal with suddenly departing elsewhere, due to being called into a battle against abyssal incursion).
Hollow is the worldly domain of Discordia, the Hierarch of Strife, established at the lowest level of the Labyrinth and surrounded by hundreds of city-states of deep goblins and numerous other species that has fallen to the corruption of Discordia. No one has truly managed to penetrate this defenses, reach the Hollow itself and return with knowledge on how it looks like and why is it called like that.
Legions of the Hollow regularly attempt to march upward, however thus far they have rarely managed to march past the Undersea region. The resistance from numerous underground civilizations, and the Labyrinth itself is too strong. However when the Hollow's history it only controlled a single layer of the Labyrinth - and now it's actually closer to three ones. The Discordia has expanded her domain to triple its original size in five centuries of grinding warfare, and she is ready to continue that slaughter until the entire Labyrinth falls.
Void is the 'worldly' domain of Ruina, the Hierarch of End, established outside of the light of the Sol, in the cold vacuum between the stars (if they even exist - nobody is truly sure, and even the mages seem to have mostly dropped that particular line of investigations). The celestial phenomenons that do not include the sun are typically observed extremely close by the authorities, as they often predate various incursions from the Void.
They typically include a very scary meteor hitting some place and unleashing something terrible, and less a direct alien invasion. Forces of Ruina are counted among the most terrible, with truly alien minds and a desire to completely annihilate everything that exists - if Ruina would have her way with the universe, there would be nothing left to deform. Merely an empty void without even the slightest bit of matter existing.
Gehenna is the Spiritual World in the relative proximity to the Prime World that has been taken over by the forces of the Pentagram at some point and is currently the main hub of their extrauniversal operations. It is considered to be akin to a 'domain' of Odius, the Hierarch of Hatred, just placed outside of the material universe for a change. Its also typically described as place that's utterly undescribable, where only the strongest of visiting mortals (typically the Chosen Ones) capable of not being driven insane merely by seeing it for a split second.
It includes the most straightforward forces of the Pentagram, the ones that can actually be summoned by the cultists of Odius into the Prime World through some rather archetypal evil magic rituals. Of course, straightforward doesn't mean weak - if anything, they are believed to be including the strongest forces available to Pentagram, at least when it comes to their pure fighting (and corruptive) power.

Military

The Eldritch Knights are the heavy-hitters of the Gehenna, towering warriors in black armours through which some undescribed (and horribly smelling) liquid is soaking through. Their armours are covered in blasphemous scripts and pictures, all of them enchanted to damage minds of anyone in the vicinity. They are send as a vanguard of the legions of Odius, driven by an omnicidal mania so typical for all servants of Pentagram - and a deep desire to break those in front of them, in body and mind.
They also possess an internal hierarchy - with some of the strongest knights being said to be fallen deities of old. Some are also made to carry banners carrying words and symbols so terrible that a simple gaze at them can shatter form of weaker mortals, causing grotesque mutations and (temporary or, if not treated in time, permanent) insanity. What all of them are in-common is that they are often summoned not only by cultists of Odius but also by the forces of the Prime World's domains of other gods of Pentagram.
The Watchers are shapeshifting infiltrators and sneaky corruptors of the Pentagram, granted sufficiently mortal-like mind in other to properly understand those that they are supposed to drive insane. They tend to appear as either pale humanoids with black eyes (and hair that appears to writhe unnaturaly) or as a grotesque mass of tentacles surrounding a single, fell-looking eye. that appears to be floating in air.
Watchers are a double-edged weapon. Their minds are vaguely human, which makes them excellent at exploiting mortal weaknesses and driving them into evil. However, they are also human enough to sometimes end up being the once that are influenced by the mortals. They are even cases of consensually born Amalthians with neither of the parents being a mortal worshipper of the Pentagram.
The Terrors are a terror weapon of Odius, Impossibly tall and elongated humanoids (with greatly varied exact size) with void for eyes, that tend to skitter at the edge of sight, continuously haunting their target until madness comes. Sometimes entire communities can be haunted that way, but when that occurs, the Terrors get creative and patient. People disappear, stolen into the Terror's pocket dimension, and their tortured bodies are left to be found in the most unexpected places. Murders are instigated, and the Terror will manipulate the people to commit worse and worse crimes as their minds crack.
To the eternal woe of the world, Terrors as described above are merely imperfect forms. Occasionally a particularly powerful Terror awakens into its true form. When that occurs, entire cities or even provinces are driven irreversibly insane, with only a handful survivors that has somehow managed to stop the Terror's rampage - only to have the memories of what they saw evaporate from their minds, the exarchs saving them from being slowly driven insane by the memories in exchange of what they achieved.
The Voidborns are mysterious starspawns that are one of the most commonly spotted servants of Ruina. They are crystal-like objects that are capable of floating around, with their size and colour greatly influencing the actual level of danger that they represent. Their 'body' temperature is that of deep space, cold almost to the level of absolute zero. They are also capable of defending themselves by laser-like discharges, but the word 'unholy' doesn't even begin to describe the result of being hit by it.
The most terrible weapon of the voidborns is the fact that they are annihilating Creation around there. Wherever the voidborn-carrying meteorite (or something else) lands, the place soon changes into a gaping hole, from which more and more voidborns emerge and continue devouring the Reality itself. When such a swarm is finally defeated, it might take decades for the wound to heal.

Religion

High Gods

Officially, the Pentagram (despite the 'penta' part) has only four high gods. It's Odius, the Hierarch of Hatred, high god of chaos of madness, and the undisputed head of the Pentagram. Discordia, the Hierarch of Strife, high god of lies and conflicts. Ruina, the Hierarch of End, the high god of destruction, genocide and omnicide. And, finally, Poena, the Hierarch of Pain, the high god of pain, blood, sadism and tortures.
What only a handful of very well-informed people know, is that there is a fifth high god of the Pentagram. However, it is yet to be fully born. This god is Nil, the Non-Hierarch, high god of atheism and nihilism. The full gestation of this paradoxical entity is a the end-goal of the Pentagram as a whole, as its birth will not only tilt the balance of power in the war in Pentagram's favour, but will also - most likely - shatter a big part of the multiverse due to said paradoxical nature of Nil.

Corruption

One thing has to be said. There are very, very few cases of someone willingly deciding to serve the Pentagram. Both because one has to be thoroughly insane to do that and because Pentagram is fully capable of accidentally obliterating willing converts with its own presence. The Pentagram forces are living disturbance of the form, capable of rendering a mortal with low resistance to deformations into a still screaming (and immortal) wall of flesh (or a pillar in a cave) merely by passing by.
However, being corrupted by the more insidious branch of the Pentagram is one of the ways of reinforcing your form. This corruption is equally overwhelming. Merely being exposed to it can and most likely will push the corrupted into an even greater acts of depravity. Brief exposure to a corrupted artifact will drive many homicidally insane, with even family members raping, murdering and devouring each other in a mindless frenzy. Those who were strong enough to resist the complete shattering of mind will descend down the slope into a more functional forms of omnicidal mania.
This is where majority of the Pentagram's cultist appear. They were broken by what they saw, sometimes in a way so minuscule that it passed under the Imperial Cult's radar - and blossomed years later. Sooner or later they start committing heinous deeds and leavings things that will eventually break some people, who in time will continue the cycle.