The Cabal Aeternus

The Helstraad Empire was ruled by the Cabal Aeternus, a group of four extremally powerful individuals. Since they ruled over a millennia ago and their empire crumbled during The Incursion, only Arborian historians are versed with the cabal's history today. It is said that these four individuals were immortal, powered by arcane magic stolen from the gods, as their history mentions similiar persons across the centuries and even millennia. Their reign is said to have ended during the Incursion and are presumed destroyed by the aberrations entering Merra en masse through the Rift.
The Incursion was a result of the cabal's meddling in opening forbidden planes, and as a consequence hordes of outworld monsters broke through to the Material Plane across Merra, but mostly around Helstraad's capital, Hel, where the centre of the Rift appeared, The cabal's mastery of grand magic allowed them to put at bay the monsters but following the Mending, which evaporated the Weave across Merra, the cabal's defence crumbled.
Merrian historians consider the cabal and its member destroyed during these years.
The Arborian historians list the four cabal members as follows:

Xadaron, Steel Caller

The Arborians history only has one record of a meeting with the Steel Caller, which occured during the summit when the conflict between the two empires was decreed as over.
It is written that he was tall and slender with rough, leathery yellow skin and bright black eyes that were sunken deep in their orbits. He had long and angular skull, with small and highly placed flat nose, and ears that were pointed and serrated in the back side. It was also written that his cruel smile revealed canine like teeth.
The document went into some detail on Xadaron's appearance as he did not share the appearance of any other race on Merra, with exception of Ihra.
Xadaron's arcane control over metals and alloys were second to none, even superior to anything the dwarves could smith in their time. Together with Ihra, he could forge powerful metallic constructs and bring life into them, or raise up fortresses impervious to everything but specific magic. These would be further enhanced by urtilising the pwoers of the other two cabal members.
The sorcerer was said to be the leader of the cabal, with a head for patient stategies.

Ihra, Flame Mistress

Like Xadaron, Ihra was of this unknown race. She was described as appearing to be younger than the Steel Caller and burned with a passion for excess and destruction. Where the Arborians write of the great conflict, Ihra is mentioned frequently as she often led the Helstraad legions into combat. She was exceptional warrior who wielded both sword and staff andfurther enhanced by her control over mundane and arcane fires.
Some rare accounts write about her devouring her enemies and drinking their blood.

Teeálya, Traitor to her Race

An elf who betrayed the Arborian Kingdom and joined the Cabal Aeternus. Prior to her treachery, she was a powerful Arch-Druid, second only to the King. She is greatly revilled in Arborian culture, and there are to this day a select few hunters who search for her even a millennia after she disappared from Merra.
A black tome that accounts her treacheries describes that Teeálya did not change in appearance with the exception of growing pale and her teeth could turn canine.
The elf's magic contol over nature was absolute, able to turn land and its creatures against her enemies. Her mastery of the bow was also unparralled, combined with twisting magical energies that would twist and mishape its target into something unnatural.

Cailus Blacktooth

Although Cailus was human, his origin of culture was never revealed, for it was written his dialect was strange, even one of Helstraad. His appearance was described as a fair skinned and older man with pitted black teeth, usually wearing a plain wizard's robe.
His main power was said to be of necromancy, able to raise the dead, yet in the great conflict against the Helstraad Empire the tactic of raising the dead was infrequent. It was his flying ships that caused the greatest threat to the Arborians and their allies, rumoured to be powered by the souls of men. Thankfully, the magical ships all fell to their destruction during the Mending, when the Weave was all but extinguished from Merra
Some write that Cailus' actual drive was knowledge and experimentation, which ultimately opened the Rift that would see the fall of the Helstraad Empire, but aso cause untold destruction and death across all off Merra.
The wizard also knew that both Xadaron and Ihra were offworlders, two of the githyanki who escaped from Darkspace and arrived in Merra. Cailus himself was an offworlder too, heralding from another place known as the World of Greyhawk.

Founded
Unknown, but prior to the founding of the Arborian Kingdom
Destroyed
Between 4245 to 4250AC, presumed destroyed by the Rift aberration.

Ihra & Xadaron, masters of fire and steel

Triptych found in the Temple of Azuth