Swamp demons are rarely the main antagonist, but they shape everything around them, don't they? The second a character steps into that misty, sucking bog, the mood shifts. You're not dealing with a clean, epic threat; it's this pervasive, corrupting presence. The air gets heavy, the light goes sickly, and every sound is muffled except for the squelch underfoot. It makes the story feel claustrophobic and hopeless in a way a castle siege doesn't. The demon itself might be almost elemental—less a creature with a plan, more a force of the landscape turning against you.
I think that's the key difference. A vampire lord has a certain elegance, a dark pride. A swamp demon? It's pure, ugly decay. It doesn't want to rule you; it wants to dissolve you, to pull you down into the muck until you're just another part of the fetid water. That kind of threat changes the protagonist's goals from victory to simple, desperate survival. The atmosphere becomes one of inescapable erosion, both of the land and the spirit. It's a grimmer, stickier kind of horror.