Zariel

(a.k.a. Lord of the First, Archduchess of Avernus)

Zariel was once an angel, but her impetuous nature and love of battle led to her fall. In her previous life, she was charged with observing the battles on Avernus and tracking their progress. From this exposure, she grew obsessed with the Blood War, and a thirst for battle grew within her that she couldn't ignore. In time, she became frustrated that she and the other angels were forced to remain spectators while the demons and devils battled. The hosts of Mount Celestia, she believed, could descend upon Avernus en masse and wipe both hordes of evil from the cosmos if they so desired.
After her repeated requests to join the fray were denied, her frustration overtook her, and she launched herself into the Blood War. Accompanied by a mob of mortal followers, she cut a swath through a legion of devils before their numbers overwhelmed her. A delegation of bone devils later dispatched to the site by Asmodeus recovered her unconscious form beneath a small mountain of her slaughtered enemies. After allowing her to recover in the depths of Nessus, Asmodeus installed her on Avernus as his champion and new lord of that layer.

A Fighting General

When Zariel supplanted the pit fiend Bel as the ruler of Avernus, that change signaled a major shift in the devils' tactics. Zariel's fiery temperament and reckless maneuvering stand in stark contrast to Bel's approach, which relied on his expertise in logistics and defensive tactics to make steady but slow advances. Bel remains one of her chief lieutenants, and he doesn't discourage her reckless tactics in the hope that she will overextend her forces and suffer a defeat that leads to her removal.
For now, though, the devils are fighting with a berserker fury under her charge. Zariel leads their offensives herself, and her unyielding resolve coupled with the fighting skill of her underlings has pushed nearly all the demonic invaders out of Avernus.

The Warrior's General

Zariel is no politician. She lives to fight, and she bases her assessment of those she meets on a combination of their combat skills and their willingness to use those skills. In Zariel's eyes, zeal and fury are as important in a fight as iron discipline and extensive training.
That outlook has made her something of a political pariah, since she has spurned overtures of alliance from other Lords of the Nine and elected to swell the ranks of her armies through the aggressive recruitment of mortal souls. Her agents offer gifts of martial skill and courage to any who are willing to bargain. Zariel needs souls that yearn to prove their worth on the battlefield.

Lady of Ruin

Zariel rules over the ruin that Avernus has become. Once it was a bustling realm filled with cities, trade outposts, and other features, but recent activity in the Blood War has reduced it to a blasted wasteland. The few structures still standing are citadels constructed by the devils to repel attackers, to be rebuilt each time the front line of the war moves. The devils are in control of Avernus at present, though the fighting goes on (as it always does) in isolated locales throughout the layer.
Adventurers able to avoid the other occupants can find treasure within some of the ruins, though they must venture to the fringes of the active conflict to have any hope of finding places that have not yet been looted. Many of the active citadels on Avernus loom over the River Styx or surround portals to other planes or to Dis. An amnizu presides over each location, directing the lemures that emerge from the Styx, marshaling forces against invaders, and preventing unauthorized travel.

Baldur's Gate: Descent into Avernus

The Companion

Zariel never forgot her betrayers—those who ran from the battlefield. She kept a close eye on Elturel, waiting for the day when she could enact her revenge. That day came in 1444 DR, when Elturel was conquered by a vampire lord. A priest of Torm named Thavius Kreeg appealed to any power to help him save his holy city. In that instant, Zariel emerged from a pillar of fire and offered him a deal, which Thavius readily accepted.
In the sky over Elturel, a radiant sphere called the Companion appeared, blazing like a second sun. Powered by a planetar imprisoned within it, the Companion bathed Elturel and the surrounding countryside in holy light which sent the vampire lord scrambling for the shadows and laid waste to his undead army. Thavius Kreeg, who took credit for summoning the Companion, was hailed as the savior of the city and rose to become its high overseer. Under Kreeg's decades-long rule, Elturel embarked on a path to becoming not just a holy city but the holy nation of Elturgard.

Thavius's Betrayal

Thavius was told the exact time when the Companion would cease to protect Elturel. In the weeks leading up to this fateful hour, Thavius urged Grand Duke Ulder Ravengard of Baldur's Gate to visit Elturel, to help settle some longstanding disputes between the two cities. Facing political pressure at home, Ravengard reluctantly acceded. After greeting Ravengard's delegation, Thavius Kreeg fled Elturel in secret and watched from a safe distance as the Companion transformed from a radiant sun into a black orb awash with crackling energy. It ripped Elturel from the Material Plane, transporting it and all its inhabitants to Avernus. A few other city folk managed to escape before Elturel disappeared. Where it once stood, only a crater remains.
Disguised as a refugee, Thavius Kreeg was one of the first of his people to arrive at Baldur's Gate with news of Elturel's "destruction." He slipped through the city's gates with the help of his influential friend, Duke Vanthampur, and took refuge in her home.

Extra info

Once a mighty angel charged with watching the tides of the Blood War, Zariel succumbed to the corrupting influence of the Nine Hells and fell from grace. Asmodeus admired Zariel's passion for war and offered her rulership of Avernus. She accepted his offer and was transformed by Asmodeus into an archdevil.
Zariel's "rise" came at the expense of Bel, her pit fiend predecessor. Zariel and Bel hate each other. To keep Bel busy and out of her sight, Zariel tasks him with forging weapons, armor, and gruesome demon-slaying machines.
To replenish her legions, Zariel needs the souls of mortals to create lemures, which she can then promote to higher forms of devils. Only by tempting mortals with power on the Material Plane and striking bargains with them can she bind them to her in the afterlife.
Matalotok. Zariel recently defeated the demon lord Kostchtchie and took his hammer, Matalotok, which grants Zariel immunity to cold damage. Without this hammer, Zariel has resistance to cold damage instead.

Alignment

Lawful Evil