Encounters: Storm Horns and Thunder Peaks - General Temperate Mountain

Will-o-Wisp (CR 6)

Small Aberration (Air)
Alignment: Usually chaotic evil
Initiative: +13 (+9 Dex, +4 Improved Initiative); Senses: darkvision 60 ft., Listen +17, and Spot +17
Languages: Common and Auran, plus two bonus languages

AC: 29 (+1 size, +9 Dex, +9 deflection), touch 29, flat-footed 20
Hit Dice: 9d8 (40 hp)
Fort +3, Ref +12, Will +9

Speed: Fly 50 ft. (perfect)
Space: 5 ft./5 ft.
Base Attack +6; Grapple -3
Attack: Shock +16 melee touch
Full Attack: Shock +16 melee touch
Damage: Shock 2d8 electricity
Special Attacks: -

Abilities: Str 1, Dex 29, Con 10, Int 15, Wis 16, Cha 12
Special Qualities: immunity to magic, natural invisibility
Feats: Alertness; Blind-fight; Dodge; Improved Initiative; Weapon Finesse
Skills: Bluff +13, Dimplomacy +3, Disguise +1 (+3 acting), Intimidate +3, Listen +17, Search +14, Spot +17, and Survival +3 (+5 following tracks)
Advancement: 10-18 HD (Small)

Climate/Terrain: Temperate marsh
Organization: Solitary, pair or string (2-4)
Treasure: 1/10 coins; 50% goods; 50% items

Source: Monster Manual

Spell Immunity (Ex): The only spells that can affect will-o'-wisps are magic circle against chaos, magic circle against evil, magic missile, maze, protection from chaos, and protection from evil.

Natural Invisibility (Ex): A startled or frightened will-o'-wisp can extinguish its glow, effectively becoming invisible as the spell.

Will-o'-wisps usually avoid combat. They prefer to confuse and bewilder adventurers, luring them into morasses or other hazardous places. When they are forced to fight, they loose small electrical shocks, which act as normal melee attacks.

Faerûnian Random Encounters by Region and Locale