Rampant Bloom

Rampant Bloom is by many considered to be among the most atrocious of the Fallen forces. In fact, only the Pentagram's presence in the area is capable of causing wider alliances of convenience between forces that until recently attempted to murder each other. That's because the Bloom's planned redefinition of the multiverse is radical and terrible, and its propensity for usage of chemical and biological weaponry (among some even worse things) sets it at odds with almost everyone.
The Bloom is best described as evolution - and nature - gone mad. Its forces seek to completely reorganize the way life works. Majority of its forces are species of beasts and animals that go beyond the meaning of the world 'invasive' - they are downright destructive. And if left alone will rearrange the entire ecosystem into a nightmare of perpetually devouring yet rapidly spreading lifeforms that are not only horrible to even look at but also deadly in numerous ways. Probably including the diseases they spread. After all, the strong lifeforms will adapt, and the weak ones (probably including majority of mortal species caught in the crossfire) will die out. Day like any other in the Bloom's world.
Unlike for example the Breathless Tide that seeks to kill everything but also sees themselves as benevolent force of change, the servants of the Rampant Bloom that aren't mindless are typically spiteful, often misanthropic and almost always exited by things that other people seek as disgusting. They also tend to know that what they are doing isn't really helping anything in a possession of sapience. In a way, the Bloom's infestation is less of a conquest and more of a natural disaster, threatening to collapse entire ecosystem and drive entire species into extinction - its' servants do not care about controlling anything, only about the blessed change that ruins the world.
In the end, Bloom infestations unite almost anyone against them. A significant presence of the Rampant Bloom forces often makes the adventurers march to war alongside the undead legions of the Breathless Tide (strengthening combat abilities of all lifeforms is rather counterproductive to its stated goal of killing all life after all). There is always a purge (often make complicated by the nature of the Lost Lands - and made simple by the Wild Courts treating the Bloom as a dangerous, corruptive force). One that seeks to completely erase any signs of the Bloom force. If even a handful of will be spared, it will soon change back into endless swarms of fanged and venomous horrors. This is especially true when they somehow spill into the territory controlled by the Grand Empire of Karadia, which lacks the Wild Lords to quickly repair the damages.

Structure

There is no such thing as 'organization structure' of the Rampant Bloom, there are only outbreaks of its forces and occasional cults. The former is self-explanatory - all you need is a handful of Bloom's lifeforms, and soon you have thousands of them, chewing through everything that moves (and most of things that doesn't), changing the previously fertile land into a desolate wasteland filled with horrible things. Even if one fully exterminated all of its creatures in the universe, those outbreaks would still occur, as the Bloom is perfectly willing to occasionally drop some creatures from spiritual worlds to spice the material ones a little.
This forces the enemies of the Bloom to use their armies (and, in case of the Grand Empire, the adventurers) for almsot perpetual, large-scale pest control. However, animals remain animals (save for some truly terrible abominations of the Bloom that are actually approaching sapience) and can hardly be considered an organized fighting force. This changes when the witch covens - the primary form fo the Bloom's cults - are operating in the area.
Witch covens are primary intelligences behind the devouring swarms of the Bloom, capable even of uniting numerous species of the Bloom's creatures - that would normally devour each other - into a singular fighting force. They are typically composed entirely of women (which is explained by the chief deity of the Bloom being a twisted fertility deity), deep under the corruptive influences of the Bloom. They are its willing priestesses, seeking to ruin the world merely because of their hatred towards it (one, once again, that is mostly forced on them by the Bloom itself). They also possess the ability to whip the less deadly of the Bloom's creatures into homicidal and fast-spreading frenzy with their presence, often changing what was considered to be a controlled population of a vermin into tide of murder reaching the horizon.
Other than that, there are no structures to speak off.

Territories

Outbreaks are the Bloom's equivalent of petty empires in the Lost Lands. Occasionally, there is an outbreak of the creatures of the Bloom that are strong enough to overwhelm local inhabitants. They carve themselves their own domain, typically corrupting any fae in the area into complete, homicidal insanity - and using their nature-shaping powers to perpetuate their own ecosystem. It often happens in places that can be locked-down and kept from the view of local powers - for example, an isolated valley or a cave system. There the creatures of the Bloom multiply and gather the witch coven necessary to guide them - and finally, a smaller outbreak changes into a much larger one.
Unlike the petty empires or the small undead tyrannies of the Tide, the outbreaks are temporary. Sooner or later everyone in the vicinity will unite against the Bloom and exterminate them like the plague they are. However, each time that happens, some creatures flee. They carve their own niche in the ecosystem - now being much less dangerous and spreading much slower - signify one more step to the successful 'terraformation' of the world to represent the Bloom's twisted ideals. And many decades later another witch coven shows up and agitates the remaining population back into ecosystem-crushing frenzy.
Endless Wasteland is the terrifying reminder of what's the end goal of the Bloom. It is a Spiritual World filled with some of the most terrifying and disgusting creatures one can imagine. Everything that moves there seeks to poison you, devour you, suck your blood, lie its egg in you or spreads deadly diseases and clouds of poisons to kill everything in the vicinity. All 'normal' lifeforms that somehow find their way into the Wasteland will eventually suffer one of the fates from the list. No exceptions.
It is located close enough to the prime world that majority of the Bloom creatures that spontaneously appear in an unexpected places come from here. Thankfully to the world, the most horrifying of creatures of this place are simply too powerful to appear in the Material World (which is the only thing that saves it from being devoured by kilometer-long omnivorous centipedes and other horrors of this place). In fact, a lot of the things that can pass between the worlds (and that also is often summoned by the witches) are local equivalent of the insects - that are still just as deadly as one can expect.
Tower of Blight is a spiritual world dedicated to Pestilentia that - if it's even possible - is even worse than the Endless Wasteland. Those that visited it and returned alive (an incredible feat that only some Chosen Ones were capable of) describe it as an impossibly large tower in the middle of the desolate wasteland where pus and infected blood rain from the skies and filled the rivers.
The lower levels of the Tower of Blight are relatively clean and safe, even in inhabited by some terrible abominations. But the higher one goes, the more alive the tower appears. The stone walls are replaced with rotting flesh, and soon one has to flounder through pus reaching up to his ankles, while continuously repelling both horrible infections and direct attacks of continuously worse and worse creatures. Eventually, all either flee or succumb.

Military

The Swarms are the most common form of offensive forces of the Rampant Bloom. There are a handful of common, often hive-like species from the Endless Wasteland that can easily build up a sizeable number when unleashed upon the prime world, and in many cases build heavily fortified nests that are often used by the witch covens are fortresses. Normally they are a vermin that is exterminated whenever possible, but with witches in the area to coordinate their movements they can change into a menace.
The most common examples of the Swarms are the flesh hornets (known for their dreaded tendency to lie eggs in their still living victims, that their 'children' devour from inside after hatching), the numerous local subspecies of way too large spiders (some reaching two digit number of meters in size, though such creatures tend to be temporary summons from the Wasteland) and the numberless (and heavily resistant to attacks) mantis ants.
Without the witches in the area, the swarms typically switch into their less-active state. When that happens, they are to a varied degree assimilated into the ecosystems (at least those of the WIld Courts, and while everyone still tries to kill them if possible, they tend to be treated as a low-priority target. They spread much slower, too. This is considered to be a survival method, one that few powers are willing to counteract simply due to never lacking higher priority targets.

Religion

High Gods

The Rampant Bloom is represented by two deities. The first one is Occasa, the Hierarch of Desolation, the high goddess of weeds, parasites and pests. She is often pictured as a beautiful young woman covered in insects that she wears like clothes. The other deity is Pestilentia, the Hierarch of Pus, typically portraited as a sickly-looking teenage girl, often carrying some particularly defacing and noticeable symptoms.
Both deities are sometimes speculated to be connected to the Umbral Bond, specifically two of the Ancients that used to be much more active before the Twilight War and even more active before the Dawn War - the Ancient of Filth and the Ancient of Fertility. Are Pestilentia and Occasa Ancients that has somehow ascended or perhaps usurped godhoods and joined the ranks of the imperial deities? The nature of the gods is mysterious and typically beyond the understanding of mortals, leaving such questions unanswered.

The Fall

The Bloom is among the factions least likely to motivate someone to join them in more conventional ways - as a result, there have to be some supernatural recruitment method to fill its ranks. Some of its members are corrupted in the more traditional sense - they are typically infertile women whose desire for children has turned into spite for those around them. When such a woman loses herself to her spite, she might look out for different methods of having children (very... different children, but such terms being twisted is par for the course when you are on the verge of the Fall) and will feel drawn to the nearest coven, who will give her what she wants... and more. It is truly a form of insanity.
However, this simply isn't enough. Majority of the populations in Karadia will recognize the warning signs of inevitable Fall to the Rampant Bloom, and will react (in a variety of ways). As a result, many creatures of the Bloom spread a particular form of corrupting curse that only affects women (and not all of them, there has to be some moral compatibility, so it typically affects women who are already of rather bad character) who came into a close contact with a Bloom creature in its active state (namely, when they are lead by the witches). When that happens, and the victim isn't purified through proper rituals, the result is typically a quick Fall - and soon the victim ends as one more witch of the coven.