The party is three hours into a bog at night. The torches are guttering. Then the ranger spots a lantern glow off to the left, and somebody says, "There's a path over there." Everything that follows hangs on that sentence. A will-o'-the-wisp that zips in and eats an arrow is a wasted monster. A wisp that talks the party off the stones of their own free will is the one worth running. So play it as a patient lure, and let the stalking and the bog do most of the killing.
What the wisp actually wants
The will-o'-wisp stat block tells you how this thing hunts. It's a Tiny Undead with AC 19, 27 hit points, and a 50-foot fly speed with hover. It resists most physical damage. It's immune to lightning and poison, plus a long list of conditions that includes Grappled, Prone and Restrained. Hitting it is hard. Pinning it down is harder.
Its offense, on the other hand, is thin. Shock is one melee attack at +4 for 11 (2d8 + 2) lightning damage, which will worry a level 2 wizard and bore a fighter. There's no area attack and no way to hit two targets. The Ephemeral trait means it can't wear or carry anything, so it can't drag anyone anywhere. It moves through creatures and objects as Difficult Terrain and takes 1d10 Force damage if it ends its turn inside something solid.
The ability that matters is Consume Life. As a bonus action, the wisp picks one living creature it can see within 5 feet that has 0 Hit Points. That creature makes a DC 10 Constitution saving throw. On a failure it dies, and the wisp regains 3d6 hit points.
Notice who that works on: only someone already down. A wisp can't burst a healthy party. It's built to find the straggler who fell in a pool out of the cleric's reach, and finish them while nobody can get there.
The folklore agrees. In English and Irish bog-light stories, the light (Jack-o'-Lantern, Will-with-the-wisp) never throws a punch. It bobs ahead of a lost traveler at walking pace, always a few yards farther on. The traveler follows it off the firm track and sinks to the knees, then the waist, and then the light goes out. One Irish telling makes it a dead man called Jack, so mean in life that heaven and hell both turned him away, left to wander the marshes carrying a single coal. Play your wisp in that spirit: a hungry, spiteful dead thing that likes the chase as much as the meal. It speaks Common and one other language. It can call out, it can laugh, and it can put on a false voice the way a mimic puts on a false shape.
Then there's Vanish. As a bonus action, the wisp and its light turn Invisible for as long as it concentrates, and the effect ends the instant it attacks or uses Consume Life. Nearly every scary moment you'll build comes from that one bonus action.
Foreshadow it long before anyone sees a light
The wisp lands hardest when the party has been half-expecting it for two sessions. Drop cues early and let them stack.
From a hilltop at dusk, the party sees one point of light far out over the marsh. It moves slowly, stops, then drifts back the way it came. Nobody lives out there. At the marsh edge the air smells of rotten eggs and wet leaves, and bubbles rise from black water with no wind to stir it. The ferryman at the last village takes their coin for a morning crossing and flatly refuses anything after sunset: "Causeway's fine by day. Night, it forgets where it goes."
Give the village children a skipping rhyme, sung in the background while the party haggles over rope:
"Light that walks backward, light on the fen, Follow it once, you'll not follow again."
Players remember rhymes. When a light really does walk backward in front of them, someone will say the line out loud, and that's your payoff.
The stat block supports the image:
The wisp sheds Bright Light in a 20-foot radius and Dim Light for an additional 20 feet.
SRD 5.2.1, "Will-o'-Wisp"
Outdoors at night counts as Darkness under the SRD light rules, which makes it Heavily Obscured. Dim Light is only Lightly Obscured. So a wisp hanging at the far edge of torchlight, or well past it, looks like a glow with nothing behind it. The party knows something is out there and can't tell what. You can hold that picture for several scenes before the wisp lifts a finger.
Spread these cues across sessions and bury them among ordinary swamp details: a heron, a sunken rowboat, a farmer hunting a lost goat. That keeps the pattern from jumping out. Our piece on how to foreshadow a twist without tipping your hand goes deeper on spacing.
Three hook variants to drop the wisp into any campaign
Each of these brings the party in on a rumor or a job. Nobody rolls initiative in the first hour.
The missing child's light
A month ago, Marta Fenwick's son Tobin went out after a strayed ewe and never came home. Every night since, she has seen his little tin lantern bobbing at the edge of the marsh. Every night her husband has to hold her back. She hires the party to bring Tobin home.
This hook makes the players want to follow the light, because following it might save a child. Let them find real traces along the way, like a dropped shoe or a tuft of wool on a reed. Decide in advance whether Tobin is dead or alive on some island the wisp keeps circling. We prefer alive. A rescue with a lure standing in the way forces a harder choice than recovering a body.
The cursed causeway
The Greywater Causeway is a line of old flat stones, the only safe way across three miles of mire. Lately, people crossing after dark report a kindly light that showed them a "shorter" route. The ones who took it are gone. A merchant guild posts a bounty to make the causeway safe, or at least to learn what's swallowing the wagons.
The strength here is that the safe path is right under their boots. The party knows exactly what not to leave, so every step off the stones is their own decision. Mark the stones on your map and mention them every round. When the players stop hearing about the stones, they start drifting off them.
The graveyard beacon
The parish cemetery at Low Barrow flooded when the river shifted. Headstones poke out of standing water, and a single pale light hangs over the drowned chapel. Grave robbers keep going in for the jewelry buried with the old landholders. They keep not coming out. The sexton wants to know why, and he's not fussy about whether the robbers make it home.
This one lets you build dread through small finds before the wisp ever shows: a rope knotted to a headstone and trailing into deep water, a lantern with fresh oil sitting on a tomb lid. Our list of creepy things to find in a graveyard at night has plenty more to scatter along the approach.
The twist: a wisp that guards instead of hunts
This variant turns all three hooks inside out. The wisp is bound. Something tied it to this stretch of bog, and its job is to keep people away from one thing. Maybe a sunken coffin. Maybe the body of someone murdered and never named.
It still lures, but to frighten. It walks travelers in circles until they're soaked and shaking, then lets them stumble back onto the safe path near dawn. It only drops someone to 0 hit points and uses Consume Life when they keep pushing toward the thing it guards. That explains the survivors who come back babbling, and it explains the dead. The Low Barrow grave robbers weren't random. They were digging.
Now the climax is a puzzle. Can the party figure out what sits at the center, and then either leave it alone or offer the wisp what it wants? Make the want specific. It might want its own name spoken by someone living. It might want an old debt settled, such as a stolen ring returned to a grave or a confession made before witnesses. Since it speaks Common, it can bargain. It can also lie.
Write the binding as a real motive. "Evil light protects ruins" gives players nothing to hold. "Old Harl Dunmore swore to guard his drowned sister's grave until her killer confessed, and the killer's grandson runs the inn where you're sleeping" gives them a mystery and a moral problem at once. For building wants like that, see how to give your villain a motivation players believe.
The trade-off is that the guardian is softer, and some groups showed up for a monster. If your players are itching for a fight, run the hunter. The guardian suits tables that enjoy talking a threat down more than cutting it down.
Run the chase as folklore horror
If the party ever gets a clean line of sight on a lit wisp holding still, the scene is over. One damage spell and an archer will finish it. Don't give them that shot.
Build the ground first. Firm footing is narrow: causeway stones, a ridge of turf, the roof of a sunken chapel. Everything else is sucking mud you rule as Difficult Terrain. Add deep pools where a character who blunders in needs a Strength check to haul free while their armor fills with water. The wisp hovers, so the mud never touches it, and it can't be grappled or knocked prone. Reeds and mist cut sight lines. And the wisp's own light is the brightest thing for a mile, so when it snaps off, everyone's eyes are useless for a beat.
Vanish sets your pace:
The wisp and its light have the Invisible condition until the wisp's Concentration ends on this effect, which ends early immediately after the wisp makes an attack roll or uses Consume Life.
SRD 5.2.1, "Will-o'-Wisp"
At the table it runs like this. The wisp drifts in, Shocks whoever's at the back, and goes dark. The party is now standing in Darkness, in a bog, with a torch or two between them. Something splashes off to the left. A light appears forty feet ahead, right where the path ought to be. Another appears behind. Which one is the wisp, and which is a lantern someone dropped last week? Let them argue about it. The minute of "the stones went that way, didn't they?" is the encounter. Hit point damage is punctuation.
Aim the danger at one person. The wisp wants whoever is hurt or isolated: the wizard thrashing in a pool, the rogue who scouted too far ahead, the hireling who bolted. It doesn't have to land the final blow. The bog can handle that. Once someone hits 0, the wisp drops invisibility to feed, and its light flares bright over the fallen body. Everyone at the table knows what happens if the party doesn't get there first. Give them a round to try.
Sound does a lot of work at night. The frogs go quiet all at once. A voice calls a character's name in the tone of someone they left behind. A gas bubble pops wetly, close enough to touch. Keep the false lights coming even after the wisp is dead, since the players won't be sure it is.
Rules questions can stall your pacing here, mostly about what ends Vanish and who Consume Life can target. LorePanic's SRD search pulls up the exact wisp stat block in seconds, so you can check a line without letting the bog go quiet.
One last trick. When the party finally finds the safe path at dawn, count the stones out loud as they walk. "One. Two. Three." Stop at four, look up, and ask, "Who's carrying the lantern?"