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I'd have the CS troops jonsides with your pcs temporaily

and I think hades out of the three

but I'd presonally go with some daemonix from tolkeen

they fit more with the mindless doom style demons

a gust of spore laden wind and a HORDE of Invid.

Nightbane!

You can cue the gates to particular types of Nightbane so maybe one gate sends out mechanical creatures, while another gate sends out insectoids and a third gate sends out centaur hybrid things or whatever you think is cool.

I am a big fan of players going "What the heck is that!!!" followed by "How do we kill it?"

Oh, and makes a few beasties immune or semi-immune to normal weaponry. Even its a creature who can set up a temporary shield or other negation.