It's so weird to me how the Devourers never got explained at all. You would have thought that weird mythic creatures who feed on anima for no good reason who inexplicably appear during an anima drought would have some sort of narrative purpose. Even more so with the whole predetermined universe angle this patch is going for.
I mean, a quick explanation that they ensure noone finds Zereth Mortis by keeping people away from the in-between, and simultaneously keep a barrier up by feeding it anima would have been fine.