It is safe to say that the overall community verdict regarding the Jailer’s story is unsatisfactory to the way it is laid out. So, why not participate in a bit of rewriting for fun? Everyone is welcome to contribute an idea.
Zovaal, the Jailer, is a soul eater of unspoken horror. It is said when the cosmos came into being, from it the shadowlands was birthed, Zovaal was the first sentient entity.
Zovaal is the true Death Incarnate, a walking definition of entropy, existing to devour souls that pass through Shadowlands.
It would be the First Ones that became aware of this unstoppable entity after discovering the Shadowlands, and while as powerful as they were, could not stave off the hunger of Zovaal.
Therefore, Zovaal would be imprisoned within the Maw, a pocket dimension residing within the shadowlands however, in order to keep this prison sustained, Oribos, the eternal city was created. It’s custodians would be the Eternals, omnipotent creations of the first ones who dictated a soul’s worth before assigning them an afterlife fitting specifically for that individual.
And as a soul existed in eternity within the Shadowlands, it would produce Anima - Soul energy that ensured that the maw remained locked.
This greatly impeded the entity Zovaal, but it didn’t stop him, for souls of great malice and irredeemable morality were drawn to him, naturally drifting into the Maw where their anima would be used to form the building blocks of domination magic and necromancy.
If a mortal harnessed, or became afflicted by either of these powers, their soul would immediately be damned to the maw, regardless of their moral compass.
And thus the prison weakens as a great many souls have found their way into the maw, Zovaal using them to break open the prison that he resides.
Well, it would seem I rewrote shadowlands a bit. But this certainly gives him a bit more character. He can be an emotionless, single minded being, but it would be nice to convey to the audience that he is the way he is because… That’s his nature. He’s quite literally death, and death just wants to consume life, that’s how it works.