Spoiler there is a realmgate connecting the mortal realms to the old world the slann tried to get control of it, but tzeentch keeps it in his labyrinth the seraphon nonetheless make sure nobody gets to the realmgate, gotrek included that's because the slann don't want the past to be changed because it risks damaging their plans for the future even back in the old world the slann foresaw the end times, they foresaw the mortal realms, they foresaw the importance of sigmar and the return of chaos again and they planned accordingly since then Spoiler gotrek gets to the realmgate anyway but ends destroying it instead make of that what you will
how else do you explain the passivity and inaction or downright incompetence of the slann through the whole timeline of the old world? or the fact the plan already took into account the exodus by spawning a golden crested cohort of skinks to pilot them? as far as I know no lizardmen has the power to decide what a pool would spawn and they work in accordance to the predictions of the old ones about the needs the lizards would have at certain points in time Spoiler it all only works if the great design already intended for the world to fall exactly then and in that way
soo.. were the Old Ones actually time traveling Slann who went back to ensure that the End Times cycle would always take place?
Yeah, I don't think the seraphon were handled very well in that latest Gotrek Audio Drama. Neat they were in there at all, but boy are our motives beyond anti-chaos unclear. I liked the story overall though.
Given that in the world that was the Slann's main plan was "remodel the planet to match what the Old Ones originally intended", with the anti-chaos and anti-skaven stuff meant to be a "step one", it is hard to imagine what the Slann have in mind in AoS. the old ones never envisioned anything like the mortal realms or the mess that is AoS, so you can't easily fall back on the older lore for ideas.