Frost vrykul

Frost vrykul are a race of vrykul that live in Northrend. They have blue, frosted skin.

Hyldnir
The hyldnir are an entirely female population of frost vrykul, located in the Storm Peaks. They inhabit Sifreldar Village, Brunnhildar Village, and Valkyrion.

Brunnhildar Village is located in the eastern area of the Storm Peaks, and the vrykul there retain their ancestral loyalty to Thorim. They seem to despise males of every race. With females they seem to be more merciful. Captured males are either put to death or enslaved, regardless whether or not they're vrykul.

Members of this population compete against each other in the Hyldsmeet, the winner of which is entitled to an "audience" with Thorim. It is unknown what happens to those that fail the Hyldsmeet. They are particularly opposed to the frost vrykul of Valkyrion, whom they consider traitors for allying with the Lich King.

In Valkyrion, located in the western area of the Storm Peaks, the hyldnir there are loyal to the Lich King. They hope to "accept the Lich King's gift" and be transformed into val'kyr. Those who are judged worthy here by the Lich King will become val'kyr. Also present in the village is a breeding area for Blighted Proto-Drakes.

Darkness past
They are the servants of Thorim, a keeper of Ulduar. In these times, the Hyldnir were the allies of the ice giants. That ended when Thorim's wife, Sif, was slain by Loken, Thorim's brother. A war between the Hyldnir and the Sons of Hodir started after Thorim brutally attacked the ice giants that were framed for the death of Sif. Thorim exiled himself to Temple of Storms in grief.

Betrayal of Valkyrion
The Scourge has sent three emissaries and they were all dismembered. However, in the village of Valkyrion, the Hyldnir there, led by Yulda the Stormspeaker, accepted the rule of the Lich King in exchange for the power to become val'kyr. They capture proto-drakes and their eggs and transform him into plagued proto-drakes. This act marked the beginning of the war between former sisters.

The Crone's bargain
Some time after, a raid of Hyldnir captured some male goblins of the K3 expedition, leaving the females. Most of them are rescued, except for Zeev Fizzlespark. Trying to rescue him, adventurers finds a female vrykul prisoner, Lok'lira. She will help get Zeev back, if the adventurers help her escape. She grants them a Hyldnir illusion that allowing the adventurer to confront Agnetta Tyrsdottar and free Zeev.

Hyldsmeet
Lok'lira wants Thorim to break out of his stupor and rally his allies to retake Ulduar. She tells adventurers that they have entered the Hyldsmeet. The Hyldnir competition to decide the Thorim's next wife. The adventurers later wins the Hyldsmeet and go to meet Thorim.

Revenge
After a short discussion, Thorim leave his throne to take his revenge on his brother. He stop the war between the races of frost.

The Secrets of Ulduar
Sif is now an illusion of Yogg-Saron used to control Thorim. After Thorim is brought to reason, she transforms into a tentacle.

Ymirjar
Located in central Icecrown, Ymirheim is populated by both male and female frost vrykul. This population, called the Ymirjar, are undead and allied with the Lich King. These were vrykul who traveled to Jotunheim and fought at Valhalas. The winners of this competition are taken to Ymirheim and become Ymirjar. The losers become vargul.

Unlike many of the Lich King's minions, the frost vrykul appear to retain their intelligence and free will, serving their master of their own volition in the hopes of gaining the Lich King's favor and being turned into higher order undead. Vargul are a more standard form of undead.

It is unknown if all, or any, ymirjar were frost vrykul to begin with, or gain that form upon undeath. Non-frost vrykul, such as Halfdan the Ice-Hearted, are "judged" by the val'kyr, but are not shown to change into anything.

Though the ymirjar seem to serve the Lich King exclusively, the hyldnir serve Thorim and believe those live in Valkyrion are traitors. It is unknown what the hyldnir think of the ymirjar.

Appearance
Frost vrykul look like vrykul, except their skin is blue and layered with frost.

Culture
The frost vrykul believe that wearing a necklace of polar bear claws will grant them otherworldly strength. They are known to use polar bears as ground mounts and blue proto-drakes as flying mounts.

Slavery
The Hyldnir raid in Northrend to turn the males of the other races into slaves. Thereby, a lot of male vrykul work in Forlorn Mine under the watch of Mildred the Cruel. They work the slaves very hard, often killing them when there is opposition. The leaders of rebellions are killed without pity.

Hyldsmeet
Hyldsmeet is a great competition of Hyldnir against Hyldnir to determine who will rule by Thorim's side.

Trivia

 * Though the frost vrykul are a race, they are not unified.