Fallen entity of great power seeking resurrection, Things-that-man-was-not-meant-to-know
: Aeons ago, the Usurper was the eldest and most powerful of the gods. Or is it of the titans? Maybe it is Ouranos or Chaos itself. It all happened so long ago that no one knows but the gods and they aren’t talking. It is believed that at the dawn of time, the Usurper attempted to overthrow the Powers of the day to rule alone. The few who know of the Usurper assume it occurred early in the gods’ ascension to power, others think it was a war between primordials (Titans).
The attempt sparked a conflict that would have killed every mortal had there been any. It ended with the utter destruction and scattering of the essence of the would-be ruler of everything. But you can’t truly kill a being as powerful as the Usurper. Its essence roams creation and gathers in pockets. Sometime in creates a pocket in the astral plane, stores itself in a powerful artefact or even in the soul of a mortal with the right disposition. Some of these pockets of essence have regained various degrees of sentience and awareness of other pockets.
The Usurper wants to reform. But by now it no longer craves power; it has gone utterly insane. It wants the end of all thing. A true end. No more cycles.
Truth: I don’t even know who could possibly know that, but the truth is that the war against the Usurper did destroy the universe and then rebooted it. It occurred before the gods, the titans, before chaos itself. The essence of all immortal beings scattered and reformed in haphazard way in the new universe, very slowly and erratically. Not even the gods truly know the Usurper but they have a kind of primal memory of the incident. This is a large part of their anti-interventionist stance. They feel in their very core what a war between deities can lead to.
: None. If it reforms, the world ends. It was a different universe, back when it was first defeated. Power was more thickly concentrated. In this new universe, too much essence is scattered over billions of mortal creature. They’d never be able to unite in time to contribute any kind of resistance.
: Destroy creation.
1-Reform at all cost
2-Cause a god to go mad and then trigger a genocidal war. Failing that, doing the same to any lesser power or even a powerful mortal is good too.
3-Spread nihilism and apathy. The less people want to fight, the easier ending the universe will be.
: Eons of being barely sentient made him stark raving mad. It still has some of its former trappings of ambition and rulership, but this is just to seduce the more craven of his followers, those who expect riches and powers once he is back in power. None who truly have its favour expect anything else than sweet oblivion from his return. These followers have been maimed by existence so badly that suicide is not even attractive; they want the whole world to end. The back stories of such antagonists should be filled with tragedies, impossible choices, loved ones lost in horrible circumstances etc.
: The Usurper has no name anymore. It has been destroyed along with the previous universe and it has forgotten it. It doesn’t even have a name for itself, for that matter. The ‘Usurper’ moniker seeped from the gods’ primal memories. It is how they think of this thing.
: Those of his followers who are coherent enough to express thoughts are often thoroughly nihilist. Check it on Webster if you have to.
: Some of its essence is trapped in ancient artefacts. Some of these were crafted by followers for the purpose of collecting its essence, some were just corrupted. These artefact are likely to drive insane anyone who is in contact with them for too long. Gathering such artefacts and combining their essence is a common ploy amongst the cultists.
: Powerful leaders of the Usurper’s cause often are possessed/contaminated by some of its essence. In effect, they are an infinitely diluted version of the Usurper itself. Rinch the Riser is one example of such. Such figures are not tragic heroes; they are engines of entropy. Killing them usually only lead to their swift reincarnation. Heroes have to look into some way to trap them or disrupt the reincarnation cycle.
End of time theology
: They often coat their philosophy into religious discourse.
: Just about every sentient force fighting under the usurper’s banner is calling for oblivion. They want to die, basically! They should be outright suicidal in their tactics and schemes, often wasting resources that could have lived to fight another day.
In a nutshell
: Tharizdun focusing on shadow instead of elemental powers with a pinch of millenarian sect.
: Nihilists cultists of all breed with the occasional tragic figures driven mad by grief that might be rescued. A core group of followers are Shadar-Kai. Anything with a shadowfell origin or the ‘shadow’ keyword is good. In fact, he should be the primary source of such monsters. Anything tainted with shadow and madness also fits, like Death Giants. Many undead work too, but stay away from free-willed undead who aren’t seeking oblivion (Vampires, for examples).