The Helm of Domination and Frostmourne being forged in the Shadowlands and the associated hints that Arthas may have been an agent of the Jailer was a deliberate inclusion, and that storyline will continue to be explored. We've already found hints of where this is going, but there are a lot of blanks spots in between their creation and arrival on Azeroth which which will be explored in Chains of Domination, but it's a mystery that's meant to unfold over time. Having recently learned that the Dreadlords will feature in patch 9.1, the popular theory that the Dreadlords were agents of the Jailer rather than the Burning Legion is becoming more and more likely.
You guys had a huge teaser in some of the Runecaver's memories of the Helm of Domination and Frostmourne having roots with the Jailer and his overall vision. Are we going to explore that in Chains of Domination, like perhaps what was controlling Arthas wasn't Ner'zul or his darkest desire, but maybe the Jailer himself?
We will continue to explore that storyline because it was a very deliberate underlying tone of what was going on. We saw that Runecaver memory where the Jailer demanded that he make those items, those weapons of domination, and we saw the after effects of that - we saw what happened when they got to Azreoth, but there's a lot of connective tissue in between that we need to fill in. We've hinted at some of it, there was a book in Revendreth that gave some hints as to some nefarious villains that might have been involved in that plot. Chains of domination will continue that storyline.