A strange glowing light appears in the swamp. Even neophyte adventurers know better than to follow it, but this will-o'-the-wisp is crafty, instead luring dangerous bog creatures to attack the PCs. The wisp hopes to feed on the energies left in their corpses.
Lighting is shadowy (note Luminous keywords from Adversaries).
A narrow, marshy animal trail (L) passes through a watery swamp. Water sections (W) alongside the main path require swimming. The map doesn't extend farther due to the presence of thick brush that can't be quickly navigated during a hectic battle. You can extend the water wider if you wish.
Sections are presented with a hex assumption; each section borders the two to the immediate left (diagonal) and two to the immediate right (diagonal) as well as the one above and below (if any).
PCs v W W W W L W L L W N-> L L L L W L W W W W W
The Muck Asps swim quietly, but everything else gives away its position as it moves about. This doesn't let the PCs see them, but the PCs know which sections to target at least (with the usual penalty effects applied).
The wisp (Hobby Lantern or Ectoplasm Bulb) floats over the water and thus does not swim. It doesn't make noise, but its glowing light always reveals its position.
The Adversaries here have an unnatural intelligence regarding tactics so long as the wisp is around, targeting carefully and Blocking borders as is useful. Once it's gone, however, any remaining Adversaries revert to instinct, attacking whatever is closest.
The PCs dispatch the monsters and may continue their travels. Reward the PCs with a Valuable each, found on corpses previously drained by the Wisp.
The PCs are forced to flee the monsters. They get away, but not before taking some rough hits, suffering 1 Injury each.
If you want to make these monsters impossible to outrun in the marshy conditions, change this scenario into a battle to the death. In this case, defeated PCs can spend 1 Karma at the start of their turns to recover 1/3 of their max HP, rounded up (and can take their full turn).
Increase difficulty slightly by having this scenario occur at night or during a thunderstorm, changing the lighting conditions to dark.
There are a number of other Adversaries that work well in a scenario like this. Consider Insect Clouds, Salamanders, Serpent Vines, a Wild Boar, Bolt Rats, Wood Fiends, Mindfang Spiders, Flamebelch Jeopards, Silk Spiders, and Triffids. You could even include a Thorn Shriek or Red Ooze in place of the Hobby Lantern if you don't want a wisp (or want it to be a non-combatant).