Breathless Tide

The Breathless Tide, like the Nine Hordes, sees some crucial part of the normal society and thinks that it has to go for the world to be as it should. Unfortunately, in the Tide’s case that part is the very concept of life, putting it at odds with about everyone else. The Tide considers the division between life and death as the source of all woes that the world has, and seeks to amend it. By force, if necessary. Exchanging the duality of life and death - one that, by its very mechanism, draws worlds closer to their end and draws on their resources - with a single state of undeath. Taking the best of both worlds and mixing them into perfection.
From its own point of view, the Tide is a power of benevolent change, one that seeks to fix the world. Unfortunately, to everyone else this ‘fix’ is lethal, resulting in the Tide facing a continuous opposition everywhere through the multiverse. Tide itself doesn’t seem to care, having long ago accepted that only a select few mortals are smart and imaginative enough to understand their goals and share them. The rest will help not as craftsmen - but as resources.
Due to that, the desired end that the Tide pursues is the world of a relatively small number of truly endless undead, their afterlife brought upon them in the Material World. Each of them surrounded by the personal court of greater undead (each of them sapient) - and the legions of the thoughtless lesser undead, obeying their masters unquestionably and forever. However the Tide is known to not oversee its warlocks too closely - they expand its influence merely by using their magic, so whether they go on a rampage to exterminate all life or simply establish their own domain with a handful of undead servants and live their quietly. There are even many cases of temporarily alliances between sorcerous overlords of the Tide and the adventurers, though typically against enemies that are simply way worse than even the Tide.
The servants of the Tide - especially those that already achieved their preferred form of immortality - are counted amongst the most cultured of the enemies of the Grand Empire - after all, you want to have at least a handful of hobbies to pursue in your eternal undeath even if your time perception is altered by your state to the point where you do not get bored of anything. Tide has its own artists, even if they tend to be dead.

Structure

The question of 'organization' among the ranks of the Breathless Tide is a complicated one. There is always the biggest fish - a powerful warlock, a lich, an archlich or a demigod - of any particular organization, sect or army. They are their organizations' strongest members, and the ones that spearhead all their movement. Such sect/army leaders are projected 'sorcerous overlords' of the glorious world after Tide's victory - while the rest follows them either mindlessly (typically the aftermath of trying to stand against such organization, having your remains stolen by the necromancers or living in wrong village) or to become courtiers and citizens of that brave new world.
This actually offers the cults and sects of the Tide a significant degree of internal cohesiveness. After all, you need to like or at least respect your future god-emperor. Of course, there are still cases of betrayals, but in the end, the Tide doesn't look favourably on them. Leaving your sect and finding the one whose leader is more to your taste is a better option than trying to kill the person whom the gods of the Tide helped with their gifts and potentially ruining some long-term operations of the Breathless Tide. But it still happens.
The exact detail of how such an organization starts is greatly dependent on many issues. The most common recruitment field of the Tide's cultist are collegiums of the Imperial Magic Guild in the middle of nowhere, filled with mages whose careers hit a dead end and have to endure living next to country bumpkins (and people who look at them with fear in their eyes). The Tide offers power to shape the world aroung you to your liking, and many fall for that. There are also much rarer non-mages cultists, typically either sorcerers or martial artists who want to piggyback that particular cult into eternity.

Culture

Contrary to popular opinions, the Breathless Tide forces - especially those that already achieved their stated goal of eternal undeath - are often highly cultured individuals. They read non-magical books. They write non-magical books. They paint. They watch paintings of the others. They sculpt. They watch sculptings of others. They do various other types of crafts, often merely aesthetic. After all, eternity might be boring.
Of course, there are also those who turn for a much less savoury enjoyments in their long waiting for the end of the world. Tide, as all other factions, is internally diverse after all.

Territories

Tide Holdouts - Unlike many other factions, the Tide didn't bother persuading the lost empires to their cause. But instead, their armies have overtaken many such countries and exterminated their population (or enslaved some of them with some more long-term corpse harvesting in mind), establishing numerous holdouts in the Lost Lands. Each of them are essentially small shards of the world that the Tide wishes to create.
There is always some warlock, lich, archdaemon or a demigod in charge, who rules over a relatively small caste of powerful greater undead (or daemons/warlocks still pursuing immortality), and a way more massive population of mindless lesser undead who is responsible for both defense and industry of the resulting small empire. Said empire will also try to expand, if only to gather more and more corpses to work on. And if a number of them (or one significantly powerful) will exist nearby, they will sooner or later amass army large enough to strike against the Grand Empire.
Black Tomb - A nearby Spiritual World with deep connections to the Prime Material World, Black Tomb is described as an endless underground maze of burial sites from every imagineable (and some unimagineable) cultures and time periods. It's a realm of the Dead, its corridors guarded by endless armies of those, supported by many immortalized warlocks, greater undead that were somehow pulverized while in their Material Worlds, and - last but not least - demigods of the Breathless Tide.
To invade the Tomb is a folly, especially as everything that dies in this halls will quickly stand back up. Entire armies were devoured within minutes merely because they thought that they fight through the labyrinths filled with undead and attack the mysterious overlord of the Black Tombs, who is inhabiting the complex of laboratories in the center of the Black Tomb.

Military

The Lesser Undead are the undead who were deemed to not be worthy of the investment of efforts and resources necessary to give them sapience. They come in four basic subtypes: skeletons, brittle yet easy to summon and useful in repetitive tasks, rotten, unhygienic but easy to command and extremely resistant to hits, ghosts, immaterial leftovers of the minds of the departed, and the lifeless - undead that were properly treated after death to stop the rotting process (very early on, if not immediately after dying) from robbing them of their movement ability, that tend to form units of elite shocktroopers of the Tide.
They are numberless - especially when the Tide cult gets to the point when they can summon finished undead from the Black Tomb - and while they lack initiative and intellect, they cannot be underestimated. They are typically used as cheap and expendable force multiplier, accompanying the more elite warriors of the Breathless Tide and limiting enemy movement, shielding the champions from deadly attacks and generally being troublesome.
There are also certain dedicated lesser undeads, much harder to create despite being essentially an upgraded forms of the four basic ones. Even some of the titans of the Breathless TIde are, in the end, lesser undead. They are made better and thus are capable of some initiative, often acting as the champions mentioned above. An example can include destroyer undead (hulking brutes of interconnected bones from many undead, armed in grotesquely oversized weapons and used as heavy shocktroopers) or the death knights (well equipped warriors made from a combination of the lifeless body of a martial artist and his own ghost anchored in a suit of armor, capable of using their pre-death abilities while being fanatically loyal to their summoners and motivated by deep desire to fight and destroy all in front of them.
The Greater Undead are sapient, to at least human-levels. They are often simply unique versions of the standard lesser undead, created with rarer resources, and by more gifted warlocks and liches. Some say that the will of Nex is involved too. The result is a more powerful version of the basic undead, with mind, sapience, and ability to (at least to some degree, depending on the state of their bodies and minds) grow stronger with training and self-alterations of the body.
There are also special types of the undead that can only be of the greater variety. There is the pale folk - an entire breed of particularly beautiful (yet deathly pale) undead with sapience, who are often seen acting as companions for the warlocks and liches with certain tastes (but are also both very knowledgeable on necromancy - and capable of draining life at will, making them useful assasins and defenders). The revenants - spirits of people who died in a place filled with magic off the Tide, feeling such boundless hatred to someone that it alone has managed to reanimate them and send them after the object of their hatred (if they died while on the Tide's side, they tend to eventually control themselves and grew stronger in power - otherwise they are mind-controled to see whoever the warlock's point to as the one they hate). And so on.

Religion

High Gods

The chief deity of the Breathless Tide is Nex, the Hierarch of Undeath, high god of undead and undeath. He is said to have retreated to his laboratory in the Radiant Worlds, seeking to create some undefined ultimate weapon that will obliterate the other high gods and finally realize his dreams of eternal undeath. There is also another god of similar level in the ranks of the Tide, however: it's Calamitas, the Hierarch of Disaster, high goddess of bad luck, natural disasters and envy.
She is said to a broken deity plagued by her own power, which renders her subject to the most atrocious of coincidences and accidents that are slowly driving her insane. Some warlocks speculate that the ultimate weapon that Nex is looking for is actually a way of killing Calamitas and changing her into a true undead, first of such kind - and thus ending her self-imposed curse. In the so-called meantime, many warlocks have learned how to channel bits of her powers to cast devastating curses on their enemies.

The Fall

The actual corruption associated with the Breathless Tide is negligible. The Tide doesn't have to corrupt anyone. It offers immortality, and many of the less religious (or moral) members of the Grand Empire are likely to follow that, in their fear of the judgment that comes after. The fear of mortality is what drives so many into the Tide's hands. After all, what it offers is enjoyable - why risk landing in a less favourable circumstances after your death?
It will also recruit some of the more prospective captives, teach them magic and twisting them through it into warlocks - which is the preferred form of the corruptive influence of the Tide. Once they are warlocks, the magic in their body will do the rest, bending their mindset (unless they possess some truly astounding willpower) into one of all the warlocks and liches of the Breathless Tide.