Did the Lost know about the Umbral Choir and are they part of their/it/him/her/whatevers plan? While I'm asking about the Umbral Choir, were the Riftborn part of the plan too? Or were they both unforeseen outcomes, because I thought the Lost were supposed to be Gods so they should have known?

Yes I know the DLC isn't out yet but the Riftborn were never revealed to have any sort of connection to the Lost, or even if the Lost knew of the Riftborn. This makes the "Plan" by the Heretic and the Lost to bring about the return of the Lost seem... well poorly planned. Making the Lost seem less Godlike and more akin to the Endless.

This is not a complaint or rant, I'm honestly just curious about the Lost and their "Plan".

Riftbron come to existence in our Universe by an accident, for sure after Endless fall. So Endless do not know about them, or their pararell Universe. 

Maybe some of them (Endless) tried to "switch" to Riftborn Univere, thus they tell us about appraing "taint" in their Univere.  <- But it is only my theory. 

I wasn't talking about the Endless, I was talking about the Lost, the Gods that the Endless destroyed to get Dust.

Polakko, Endless and Lost are separate species. Lost predated Endless, they created Dust, become (or always were)  one with it and were genocided (harvested) for it.

Rift into Riftborn universe was created in a catastrophic event by Aurigan Lost aboard Hissho's ship "Grey Owl", not long after events of quest victory in Endless Legend. You can witness this event in ES2 teaser and in Cravers, Riftborn and Vaulters prologue videos.

TommeThunderbolt, I don't think Lost were actual-all-knowing Gods, but they for sure had godlike abilities in this universe, as they could directly control quantum medium and thus fabric of spacetime. And regardless of if they knew about Riftborn and UC, ultimately their addition to galactic stage only helped to gather and control required amount of Dust for Lost resurrection.

Seems like the Lost were exclusive to the "endless galaxy" and the Umbral Chior only came over to the galaxy of the endless when it heard the deaths of the lost. I guess it took them a while to get there. I dont think the lost knew about the UC, but the UC sort of knew about the lost, or could hear them in some way at least. That might have worked in both directions who knows. The Riftborn I think nobody knew anything about until they popped out of their rift