Will-o’-wisps are shockingly (oh God, I just realized the pun there and now I hate myself almost as much as you do) nasty opponents. AC 26, Dex 29, natural invisibility, immunity to most magic…there’s a reason these things are CR 6 foes. If your party is underpowered or bad at problem solving, you’d better hope you don’t run into a string of them.
Plus, pretty much no will-o’-wisp encounter occurs without a complicating factor. When was the last time you tackled a will-o’-wisp that didn't also involve a bog, quicksand, or some other nasty hazard? And because they feed on fear, they're born combo monsters. Haunts are mostly harmless…until a will-o’-wisp shows up. A pride of krenshars won’t scare most PCs, but your hirelings and animal companions are another story, and then the wisps gain fast healing. Mothman plus will-o’-wisp plus mudslide? This just became a CR 8 or 9 encounter.
And what exactly are they? “Immortal” plus “aberration” basically makes them “anything you want.” I always default to Western Europe, so I tend to treat them similar to fey, luring travelers astray in the classic fashion. But why not make them creatures of the Plane of Air? Creations of the aboleths or caulborn? Otherworldly visitors? Living shocking grasp spells? The illuminated fragments of a computer virus? The genii loci of swamps that fester into evil? All these possibilities are on the table.
Despite my own Celtic leanings, these will-o’-wisps go a bit astray from the folklore…
Sailors fleeing sahuagin are driven toward a strange island. But the moment the ship crosses the reefs and reaches the shelter of a lagoon, the shark-men flee in a panic. The reason is soon clear—saint’s fire plays over the mast, then resolves into a string of deadly will-o’-wisps. That, and the fact that the island is covered in bluish vegetation and mi-go can be seen flitting around the peak in the distance, all seem to indicate all is not normal on this island.
Will-o’-wisps follow mothmen like pilot fish after a shark. When adventurers seek to raise a sunken tower from the bottom of the swamp, a mothman arrives to observe—possibly heralding calamity. But while he makes no move against the adventurers unless attacked, the will-o’-wisps that follow him are not so patient.
After proteans rip through an axiomite metropolis, their very presence introduces chaos into the glowing equations that are the foundation of the city—and its people. Some of these rogue, unsolvable equations coalesce into sparking, evil intelligences that lurk in the swampy, poorly proven substrata below street level.
—Pathfinder Bestiary 177
I mentioned Dragon Magazine #138 the other day. If you found a copy, flip to “Between Lightning & Thunder,” Nancy Varian Berberick’s amazing story about a dwarf and the human boy he adopts vs. a wight and some nasty will-o’-wisps (though the story calls them wood imps).
Lots of reader comments to get through…but that’s what the weekend is for, right? Have a good one!