So, my party failed to stop a summoning ritual a cult was performing because they did not discover it before it completed.

Because the party is only lvl 3 though, and for most of them it's their first game, I wanted to have a fight that showed that the world does move along with or without them and that the cult they are beginning to discover is a bigger threat than they think. (I have a way of conveying this through the story and an important NPC that the party knows)

I was planning to have them fight a demon the ritual summoned and some underling demons, but i'm not sure what demons I could use that would be a really tough challenge for them and good for RP since the cult is trying to bring an army of demons to the world, without ending up a TPK.

The party is a size of 8 players, I can provide more information on the individual party members classes and subclasses if that would help.