also the oil-black thing on the plane of the dead was one of paloma's visions. the other option was grey nothingness, i think?
We don’t know this for sure, but I think that Paloma’s vision was showing Taako two possible futures for the material plane.
One was engulfed by the Hunger, so that could be the tar one. The ash one…well, I have a theory about that.
I fully believe the Red Robes are not evil. That they are a group of people that, like the Bureau of Balance, are trying to save the world. After all, Magnus (and by association, probably the others) apparently have some connection to them. They seem to want to stop this world-devouring hunger.
The Bureau of Balance, similarly, has good intentions. They’ve seen the destruction the Relics caused and want to prevent it from ever happening again.
Both of them are going to fail. Because neither of them knows the whole story.
The tar future represents what will happen if the Bureau succeeds–the Relics are destroyed, the Red Robes are stopped, and there is nothing that can prevent the Hunger from consuming every last plane in this, the last world where life still blooms.
The ash future represents what will happen if the Red Robes win…maybe a world destroyed by war over the relics?
That is the “terrible choice between two options that will determine the fate of reality itself.” In order to save reality, to preserve life, our heroes will have to take a third option.