Surprisingly little is known of, what came to be called, the Anti-Alicorn Alliance (That wasn't their official name but occasionally they went by it)
The congregation known as Omnipotency was anything but. By the time they'd formed as one de facto alliance, they had little in the way of resources and dwindling horse-power, losing more ponies to defection than in pitched battle with Equestria.

Three key states are most documented by histories.
Marchion- Ruled by the House of Sanguine who, at the time, possessed the oldest and noblest bloodline in Equestria and saw the worship and crowning of the alicorns as high treason and heresy. Their influence was vast and it was their influence that Equestria set about shutting down, the Bearers of the Elements and their other prominent war-leaders shutting down their foundries, overrunning their strongholds and taking control of their agricultural sites. Before they could effectively play their cards, King Roth Sanguine was forced to hire mercenaries and lost them when his revenue dried up entirely.
Wise enough to know when a cause was truly lost, Roth surrendered and Marchion has endured to this day as a vassal state.
(It is believed Roth's bastard son, Baldric, defected to Equestria and gave them the idea of going after his resources. He is quoted to have once said 'Don't you worry, your highness. I have a cunning plan'

Boulea- Ruled by a married pair of siblings of unknown origin, Superbus and Lucretia. As belligerent as they were arrogant, the two were among the most vocally opposed to Equestria and repeatedly stood against them in the field with few remarkable successes. As the war started to turn against them, they sought out allies, a rare thing and difficult.
One such ally came in the form of Tirek, the approaching Barbarian Warlord. Superbus and Lucretia, in a great moment of folly, invited him to Boulea and then proceeded to order he submit to them in exchange for a subordinate position in Boulea's auxiliary armies. At what point they regretted their decision is uncertain but Tirek killed them both for their insult, pillaged the city, then went on to attack Equestria himself.
Boulea, now named Boulette, has since been a Vassal State, ruled by semi-autonomous senatorial authority.

New Hycarion- The greatest of all follies that has ever graced Equestria, New Hycarion was documented by Equestrian sources and is remembered in history simply for how absurd the state and its ruler was.
Its ruler, rather than declaring the foretold alicorn monarch a fallacy, instead insisted said messiah was he. Dvinius, a young scion of an unknown royal household, dressed himself in giant prostethic wings and five fake horns and took to calling himself Potentate Magnificence. His was a reign of never-ending idiocies, acts of spite and strokes-of-ego with little to no success against Equestria, eventually driving 'New Hycarion' to a meagre collection of sycophants and slaves living underground in a makeshift hall.
Until, it is recorded, a mysterious war leader known simply as The Warmonger offered his services to Dvinius, overthrew him, raised an army of colossal strength and size and very nearly brought Canterlot to its knees.

Nice idea you have here. Hasbro should definitly hire for for their chraracters

6000626
Glad you think so.
Though I'm not sure how long they'd want someone so obsessed with making it dark and edgy on their team.

6000693
Well, In my option, your villians Surpass Tirek much.

6000621 Love your worldbuilding as always, Purple m' dear.

6000716
Well, maybe in cunning.
But I wouldn't fancy Cascadius's chances staring up at Tirek's falling hoof.
Something I'm trying to avoid is shunting the canon villains to second-rate. No-one likes that.
Well, he's better in some factors but you're not going to see him wiping the floor with Discord and Chrysalis anytime soon.

6000726
Thanks, Wolf.
Any of them catch your eye?
I've included a rather subtle meta-reference.

6000758
I understand. But if they need a villian that shall surpass him, I think they should ask you for that.

6000763
Glad you think so.
What I'm trying to emphasise is that Cascadius has been biding his time and keeping a watchful eye.
He hasn't just 'returned' like Discord, Nightmare Moon or Sombra. He's patient.
He doesn't see the Bearers as 'meddling whelps' nor as toys to be played with.
He sees them as a significant threat and he goes about dealing with it.
He's dangerous because he's taking things seriously. Everything he does, he does to build up to his grand plan.
What that is? Wait and see...

6000769
Indeed, greater numbers don't mean victory, the right plan and tactic is critical for victory