Abreha Mirembe is the current High Lady of Aksum and an empress-claimant of Praes. As a Claimant for the Name of Dread Empress her regnal title is Sepulchral. She is the leader of a Praesi faction currently engaged in a civil war against Dread Empress Malicia.
She’s shockingly cold-blooded, even for a Praesi. Catherine Foundling noted she was emblematic of Praesi upper nobility. She’s been described as “cunning, even brilliant in some regards, but also appallingly callous. Abreha Mirembe does not have ideals – or perhaps it might be more accurate to say that her ideal is the acquisition of power no matter the costs”.
Appearance-wise, she's extremely old with dark skin that's wrinkled like a goblin’s. She must be very old indeed as Praesi nobility uses rituals to keep themselves young. She’s a Soninke. Abreha does not have a reputation for military talent.  
The Mirembe family has been ruling for over a thousand years, roughly on par with Sahelians in terms of prestige. Their seat of power is Aksum, one of the main cities of Praes.
It's unknown when or how Abreha inherited the title of High Lady. It is however known that she was already in the court when the Black Knight, Amadeus of the Green Stretch, said “Tremble, oh ye mighty, for a new age is upon you” as he declared for Dread Empress Malicia against Baleful.
Her heir was her nephew - Fasili Mirembe - who later on joined Akua Sahelian in her rebellion. She didn’t think much of him, saying he was annoying, and didn't care to bargain with Catherine Foundling to not have his soul extracted until Catherine threatened to deliver the soul to Black, who could use it to lay claim to Aksum.
As a High Lady
She first appears in the story when Catherine bargained with her for Fasili’s soul. Abreha eventually agreed to back the petition for establishing a ruling council over Callow, and further offered to withdraw support from another petition if Catherine killed the other Trueblood hostages taken from then-Heiress Akua. In the end, however, she had to settle for just having the soul of her heir spared.
In her position of power, once it became clear Malicia had started dismantling the Truebloods, she switched allegiance to primarily backing the Moderates. It’s implied she hedged her bets a little, even if she no longer supported the Truebloods overtly. She eventually disinherited her nephew and broke with the Truebloods in full during the Second Liesse and the rise of Diabolist.
After Thalassina was devastated during the Tenth Crusade Malicia’s support became shaky. High Lord Idriss had been her political ally for decades and he was also being set up as a counterweight to Abreha, who was leading the Moderates. The sacking of Nok and the destruction of the only other seaport of the Wasteland dealt crippling blows to Malicia’s prestige, which had already been steadily eroding under the constant Ashuran coastal raids. Almost overnight, Abreha Mirembe became the second most powerful person in Praes and pressure was mounting at court for her to be named Malicia’s Chancellor.
Once Foramen was taken by goblins, Abreha capitalized on her strength by secretly spreading rumours that the foreign attacks were being used by the Empress to eliminate the High Lords entirely. The High Lady also prepared several petitions to be presented at court, some of which would cast her as being attacked by the Tower. Rather than attack her, though, Malicia changed the petition to grant Abreha an Imperial Governorship of the Blessed Isle and assigned her a task to deal with Thalassina’s refugees - as a sign of trust from the Tower. In reality, this was to pit her against Catherine Foundling and the Kingdom of Callow in general.
A year later, Abreha lost significant political capital. The Legions-in-Exile had slaughtered the Aksum forward parties and driven the refugees back to the Blessed Isle and the nobility as a whole believed the enterprise was Malicia’s ploy to humble the High Lady of Aksum. Abreha tried to exert her control by ordering the lords and ladies sworn to Askum to refrain from attending court until she was summoned back to Ater though this backfired and her orders were unheeded by the most powerful of her vassals.
When Abreha finally rebelled, the initial wave of support was not insubstantial. A chunk of the former Legions-in-Exile joined her, though most stayed independent under the command of General Sacker. Along with her own High Seat, Nok also declared for her. Many suspected that Black was actually the power behind Sepulchral though this later turned out incorrect.
High Lord Dakarai Sahel of Nok remains her main supporter. This is rather considerable as with Thalassina destroyed, Nok is the only remaining major Praesi seaport. The alliance is strong: his daughter Hawulti is supposed to wed to Sepulchral’s appointed successor to the High Seat of Aksum, her grand-nephew Isoba.
As Sepulchral, Abreha understood very well that her position was tenuous at best. She remedied that by cultivating her usefulness to both the Grand Alliance and the Kingdom of Callow, often passing information unavailable to either. She also requested to open formal talks with the Grand Alliance and Callow though so far they have been informal and secret. So much so that instead of using her name, Cordelia and Vivienne both refer to her as “our friend out east” or similar euphemisms.
At the end of Book 6 it's revealed that the civil war was maintained by Dread Empress Malicia to make Praes appear less threatening to the Grand Alliance.