Fortunately, neither Cook nor Holmes gives any stats for the will-o-wisp, so I am left to my own devices. The Wikipedia entry on Will-o'-the-wisp provides this rich piece of folklore:
An Irish version of the tale has a ne'er-do-well named Drunk Jack or Stingy Jack who makes a deal with the Devil, offering up his soul in exchange for payment of his pub tab. When the Devil comes to collect his due, Jack tricks him by making him climb a tree and then carving a cross underneath, preventing him from climbing down. In exchange for removing the cross, the Devil forgives Jack's debt. However, because no one as bad as Jack would ever be allowed into Heaven, Jack is forced upon his death to travel to Hell and ask for a place there. The Devil denies him entrance in revenge, but, as a boon, grants Jack an ember from the fires of Hell to light his way through the twilight world to which lost souls are forever condemned. Jack places it in a carved turnip to serve as a lantern.This story is not found in Europe, however, where will-o-wisps were thought to be spirits of the dead or the supernatural. Thus, my version will fall somewhere in between.
Jacques de la Lanterne
Rescue me from the cruel sword, and deliver me from the hand of aliens, whose mouths speak lies, and whose right hands are false. — Psalm 144:11
Number Appearing: 1
% in Lair: nil
Armor Class: 2
Hit Dice: 1+1 (turn as 2 HD)
Attacks: see below
Special: see below
Hoard Class: nil
These malevolent and mischievous spirits only appear at night or in darkness and from a distance. Those who set eyes upon them will see someone vaguely familiar holding a lantern [in the environs of the Chateau des Faussesflammes they often appear as the Pattern Juggler]. Those who show any kind of curiosity about this vision must make a save vs. spell. Those who fail will fall under a charm and will insist on running after the Jacques de la Lanterne for what they will perceive as 1d6 turns.
During the chase, the Jacques de la Lanterne will keep its distance and never willingly engage in combat. If forced, its attack will do no damage, but will paralyze its victim for 2d4 turns unless a save is made.
While the chase is going on, no mapping may take place and characters will become disoriented. At the end of the 1d6 turns, the Jacques de la Lanterne disappears and its victims are released from their charm. This chase, however, does not take place in normal space or normal time. Those characters who participated in the chase (even those not charmed) will end up in a random section of the dungeon (roll a random room or geomorph on the same dungeon level) or a random adjacent hex in the outdoors. The chase will take the following amount of time for each turn the chase goes on (roll a d4):
Jacques de la Lanternes are undead and may be Turned (if the cleric manages to make the saving throw). If turned, those who are charmed receive another saving throw. They are unaffected by the spells charm person, sleep, or hold person.