Ooooo ..... the swamp lands of Eberron.
Have you read Brooks' "The Sword of Shannara"? The part when Shea, Flick, and Menion attempt to navigate the Black Oaks? They end up fighting for their lives against tentacles that strike at them through the misty-darkness. They never see the monstrosity attacking them, but they know it means to eat them.
Something like that.
Use the concealment (darkness/fog) rules to disorient them (fog disrupts even Darkvision). Have their footsteps echo making it sound like they're being followed. Put old spider webs in their path so they get the sensation of something crawling on them. At random have small muddy pits that characters can sink into waist-deep. Nothing too challenging. You just want to raise the threat level slowly.
I would throw large spiders at them: they can't see them and the spiders know it so there is no frontal attack. Each round 2-3 spiders attack with ranged touch attack webs looking to entangle the PCs. The terror for the players is not seeing where it's coming from as the spiders use tactics.
Or maybe a team of dopplegangers that use concealment to infiltrate and attack the party using deception. They look like the other characters, right?
Doppelganger: "Take my hand, I'll guide you through"
Player 2: "Okay, let me sheath my weapon ..."
GM: *grin*
Just remember monsters should never attack the first round. Let them get into position and attempt to gain advantage, just like the players do.
