Before anyone reads this, you should know that it references...that fic with the Caribou.
It ends badly for them, let me just assure you of that.
If you really don't care for this sort of thing, understand that this is basically little more than a reference and a character's backstory. Not a whole lot of this will affect the story and no mention will be made of the unspeakable acts that took place.

A lot goes in the Frozen North but a particular tale shines through in recent years.
The native caribou are normally thought of as little more than raiders but in their own lands are surprisingly organised, tending to farm and family when not at sea. Both males and females can take up both the raider's axe and the farmer's trowel as they please and they measure strength not just at how well they slay their foes but how well they care for their loved ones. After all, caribou themselves must protect their lands and subjects from raiders far more often than their foes.
They are led by a conclave of Jarls, regional or island monarchs, with a High King (Male or female, the title is the same) chosen at a Moot after the previous king's passing, each one declaring their wishes for cariboukind and bringing proof of their right to rule with treasure or trophies.
Several decades ago to the modern day, a Moot was declared where a certain, recently-appointed Jarl nominated himself.
His name was Drainn, a prominent reaver, feared by many.
He proposed a new way of life for the caribou.
Total male supremacy, subjugation and dominance of every doe, enslaved for every male's pleasure night and day, and every fit and able male sent to the shores outside of the caribou domain to raid and bring back more females to be indoctrinated in a similar manner.
He was swiftly declared a dangerous and sick-minded fugitive and forever banished forthwith from the caribou domain, running for dear life to escape the wrath of the most prominent Jarl, Mathilda Gildenshield, who swore he would lose his head the next time she heard him speak in such a way.

Gathering a sparse amount of followers, he fled the realm and landed on a collection of craggy, mountainous isles, leading into an icy silt that spread on to the lands of ever-winter where nothing grows and no-one thrives.
Declaring himself King of what he called the Isles of Brunstr (Loosely translated to mean a beast's sexual urges and potency), he put his depraved rule into practice, a peaceful collection of Selkies the first to fall victim to the perversions of his petty kingdom, subjected to every form of befoulment until they went into shock and were then eaten.
Drainn vowed time and time again to bring Equestria to its knees and personally make Princess Celestia his pleasure-slave.
Their raids never got further than beyond the shore, the Royal Guard and sky forces pushing them back with little to no losses.
The Brunstr tribe, evidently, weren't known for their sense of strategy and were driven from the pony domains every time they surfaced.
Drainn therefore, made do with victimising any female closer to home, gathering slaves from the shores of any unsuspecting realm.
Until one particular slave caused his tribe great duress.
A doe who fought off any buck who tried to have their way with her, her hooves and teeth lethal weapons, slaying three raiders and leaving dozens more with gruesome injuries.
None knew where she came from or what drove her thus.
No spell or torture broke her resolve and so Drainn declared her 'untameable' casting her into a gorge.
It is said that the doe survived the fall and found herself in a labyrinth of monsters, all of which she fought through as she had with the Brunstr.

By whatever means she did so is unknown but after a decade, she returned in a ship with mithril plates and cannons, bringing down the towers and forts of the Brunstr, an army of fiercely loyal stags and does, armed to the teeth, and a menagerie of hellish creatures, hungry for blood.
The Brunstr fell within the day and the doe who brought them to fetlock took up a new name.
Angrboda, named after the Jotnar Giantess who, according to caribou legend, birthed the beast that would swallow the earth, the Herald of the End Times.
And it was said that the seas around Brunstr turned a deep scarlet as every stag who served Drainn and followed his ways was subjected to a gruesome death, their own slaves ordered to deliver it upon them, most needing no convincing.
(It's worth mentioning, however, that if any slave could not bring herself to kill her master, she was tied to a boulder and thrown into the sea. Angrboda clearly had no sympathy for those who would not fight back as she had)
But Drainn himself was Angrboda's alone.
A poem by Strilda, a snowy griffon in Angrboda's crew, goes as follows.

Since then, Angrboda has been a subject of both praise and fear in the Frozen North. Choosing the life of a mercenary leader, leading her fleet over the far seas.
Faring better against Equestria than Drainn ever did, her Jotnar Legion excel in anti-magical weaponry and artillery, forcing foes to fight them head-on, hoof to hoof, rarely ending well for them.
Any who know her pray they will never be forced to fight on the opposite side for it will surely be the last battle they ever see.

You better don't fight her if you want to live