The dark pact Sylvanas struck with the val'kyr forged her bond with the Jailer. Through the years, these winged sisters faithfully served the Dark Lady, even if it meant sacrificing themselves on her behalf. Now Sylvanas calls her nine val'kyr to fight for her one last time.
- Dungeon Journal
Centuries ago, when Odyn first decided to create Val’kyr, no one volunteered to be the first, so he decided to force the role on mortal Vrykul. Helya, who was like a daughter to Odyn at the time, strongly objected to this, and even threatened to leave if Odyn went through with it. (In fact, an early version of this questline even had Helya address Odyn as "father".) Seeing her threat as an act of disobedience, Odyn forcibly transformed Helya into his first Val'kyr instead, and then made her do the same to unwilling Vrykul herself.
Odyn, Helya, and the Jailer's Eye - Chains of Domination Story Background
Sylvanas: You know the truth. Nothing is fair. Not life, not death.
Sylvanas: So we're going to tear it all down.
Anduin: And what purpose will that serve? Everyone suffers, Sylvanas. But destroying everything will not take away the pain.
Sylvanas: Ah, you misunderstand. We're breaking a system that has always been flawed, and remaking it into one that is just.
Anduin: Do you expect me to believe that, after all this time, you've been fighting for justice?
Sylvanas: How can I convince you? From our first breath to our last, every decision is made for us. Then the afterlife decides what eternity we must endure. We can't even choose who we... We couldn't control anything. But through the Jailer, control of our fate will at last be possible.