Will-o'-Wisp

A wisp on the party's side is rare but not impossible. The framing that works is a bargain made in a marsh at midnight, or a wisp bound to a relic the party recovered, or a corrupted soul the cleric has agreed to escort to its rest in exchange for one task. Wisps speak Common plus one other language, so the negotiation can happen on screen, and the wisp is articulate, sly, and absolutely not on the party's side morally. It is helpful for one specific reason and will leave the moment the reason is met.

In a fight, the wisp is a scout and a lantern that can also kill. Send it through the wall (Incorporeal Movement) to map the next room, then have it return and report. In combat itself, Shock at +4 for 2d8+2 lightning is a respectable third-tier touch attack, and Vanish keeps the wisp alive between turns. Don't put it on the front line; AC 19 is good but 27 HP is fragile against any focused fire. Let it pick off enemies the party leaves at low HP, since Consume Life is genuinely useful to a wisp ally as long as the players accept that it's eating souls.

The wisp will not protect a downed PC. If a party member drops, the wisp is calculating whether to Consume Life on the friendly target instead of the enemy. Make the temptation explicit in narration. The cleric who notices the wisp drifting toward the bleeding rogue should have one round to react. Once the bargain is fulfilled, the wisp says something quietly cruel, vanishes, and is not seen again.

Have the wisp narrate one true thing the party didn't want to know on the way to fulfilling the deal. The cost of the bargain isn't the favor; it's the information.

A will-o'-wisp is the bestiary's best ambush predator dressed as a CR 2 mook. AC 19, +9 Dex, 50 ft. of hover, immunity to Lightning and Poison, resistance to Acid, Bludgeoning, Cold, Fire, Necrotic, Piercing, and Slashing, and Vanish as a bonus action. The party will roll initiative against a glowing ball, watch it disappear, and spend two rounds swinging at empty marsh air. That's the encounter.

Lead with the bait. The wisp sheds Bright Light in a 20-foot radius, which it uses to lure parties off the safe path through bogs, fens, ruins, graveyards, anywhere there's something worse than the wisp waiting at the end of the trail. By the time the party realizes the friendly lantern is moving away from them, they are knee-deep in something they can't see the bottom of and the wisp is leading them to deeper water, a dire wolf den, a cliff. The fight starts when the trap closes, not when the wisp shows up.

In combat, hit-and-vanish is the entire pattern. Shock at +4 for 2d8+2 lightning, then bonus-action Vanish, then move 50 ft. through a wall (Incorporeal Movement treats objects as difficult terrain), then resume Concentration on Vanish next round to set up another strike. AC 19 against melee fighters with no magic is brutal; the rogue will miss more often than she hits. The damage isn't the threat; the dragged-out fight is. By the third round the party is exhausted, low on resources, and probably standing in the actual encounter the wisp was leading them to.

Consume Life is the closer. The DC is 10, which sounds trivial, but the trigger is a creature already at 0 HP within 5 feet. A downed PC that the cleric hasn't yet stabilized fails the save and dies, and the wisp regains 10 HP. Use this once if the dice line up; the table will remember it for years. The wisp does not flee; it Vanishes and waits for the party to leave or to drop someone else.

Run the wisp from behind the GM screen. Track its position on a private map. The party should never know exactly where it is, only where it just was.