My party tried to get into the Inn at Dineval, so they stumbled into the sub plot.

But aside from that...

I made the party encounter a Duergar patrols as a random encounter either in the wilderness or in ten towns.

At first they would encounter them less often, scouting missions, I had the party encounter abandoned dog sleds and have ppl from the town reporting that their family members went missing. Abducted by the Duergar for interrogation.

As the story progressed, the Duergar presence escalated with more Duergar getting into place for the attack on Ten Towns. Added small Duergar higeouts and also regular patrols.

Tried to go for the sense that "There is something going on here" and the party investigated.