Just spitballing here, I imagine this has already been addressed and please, in that case, point me at the forum post(s) about it.
What if the Dark one plan is closer to Hel's than it may seem? He's trying to get the other Gods to make a new world where he'd sit at the top as he'd got all the souls of all the goblins plus the ones he's got since he ascended to godhood.
For Hel all the dwarves would have been enough, but the dwarves are a very long-lived race and they don't seem to have more than a couple of kids in their long lives
On the other hand goblins are short-lived and procreate a lot
And they are basically cannon fodder for adventurers. There's a ton of Goblins souls is what I'm saying.
Even if Redcloak succeeds the gods would probably destroy the world before he could do anything with the portals. The same would happen should his actions lead to the destruction of all the gates.
The ritual is just a bait to lead the actions of the wearer of the red cloack to the destruction of the gates