I don’t get why they are here. (besides reusing some mobs)
We spent months talking about how essential the covenant seals are for getting to Zereth Mortis and the devourers just randomly open tears. And no the veil wasn’t weakened or anything as we still had to recreate the covenant seals to be able to follow Zovaal.
Actually there is a quest chain from the Jiro on this. Essentially, the Jailer’s domination magic stuff has the side effect of creating weaknesses in the barriers that keep devourers out. That’s why they appear in Zereth where his magic has been at work.
I mean, the writers actually didn’t botch that one, yet.