In my earlier series on undead creatures, I skipped over the will-o’-wisp, the “devil lights” of swamps, marshes and desolate battlefields. In building will-effective o’-wisp encounters, it’s necessary to bear in mind the prime directive of horror: fear of the unknown. To create suspense, it’s best never to name the enemy that the heroes are facing, and to keep them in the dark about what it can do for as long as possible. The No. 1 way to spoil a will-o’-wisp encounter is to tell the players they see will-o’-wisps.
Will-o’-wisps are like fantasy UFOs: they can bob and hover in one place or move up to a zippy 50 feet per round. They’re immune to exhaustion, grappling, paralysis, poison, falling prone, restraint, unconsciousness and lightning damage, and they’re resistant to physical damage from nonmagical weapons along with several types of elemental damage. They have darkvision out to a range of 120 feet but shed their own light out to a range of between 10 and 40 feet, although they can also wink in and out of visibility.
Will-o’-wisps have no physical attack. Their Shock attack is a melee spell attack (Wisdom-based, by mathematical inference), and against unconscious opponents, they can follow it up with the nasty Consume Life feature, which has the potential to kill a player character outright. However, between their many resistances and immunities and their Dexterity of 28, which gives them an armor class of 19, they have nothing to fear from a melee attacker. They’re the rare high-Dex, low-Strength, average-Constitution monster that isn’t a ranged sniper and doesn’t need or even want to be.
The first trick for them is to lure victims into coming into contact with them. This is better accomplished when the area is highly obscured by fog, mist or darkness, and best of all when the PCs are in unfamiliar territory and unsure which way to go. The will-o-wisp’s modus operandi is to move like a person out walking with a lantern or torch, in order to attract the PCs’ attention and get them to investigate.
When characters get close enough that they might be able to identify the will-o’-wisp as nothing more than a ball of insubstantial light—figure about 60 feet, since this is the standard range of PC darkvision—it uses the Invisibility action, which you can assume is always Readied so that it can be used as a reaction, and vanishes. Alternatively, it may drift away from the investigating PCs, luring them toward quicksand or some other hazard. (Relevant fact: In real life, as opposed to the movies, you can’t really sink into quicksand and be suffocated. It’s dense enough, and you’re buoyant enough, that you’ll stop sinking when you’re about waist-deep. But you can get badly stuck in it. A character who steps into quicksand may be considered restrained, and the more he or she tries aimlessly to get out—as opposed to relaxing, lying down and patiently backstroking to safety—the harder it will be to pull him or her free.) When a PCs is restrained, prone, incapacitated, blinded by darkness or under attack by another enemy, the will-o’-wisp moves in swiftly and attacks. It attacks the same target again and again unless and until it’s driven off (see below) or the target is dead.
A PC attacking a will-o’-wisp isn’t blinded by darkness, because the will-o’-wisp sheds its own light, so he or she won’t have disadvantage when attacking unless it’s the result of some other debilitating condition, such as being restrained. However, unless the PC wields a magic weapon, it will be difficult to land a hit. When a PC does score a hit, whether with a weapon or with a spell, the will-o’-wisp goes invisible, then moves away, choosing a different, more immediately vulnerable target on its next turn.
When a PC is rendered unconscious—either by a will-o’-wisp or by another attacker—the will-o’-wisp will use its Consume Life bonus action. Even if the PC makes his or her saving throw, you should still describe this in horrible terms (“Dazed, unable to move or speak or act, you struggle weakly against the will-o’-wisp as you feel it try to suck the life from your body . . . ”), to drive home the urgency of rescuing that PC before he or she fails.
Will-o’-wisps’ Wisdom is high enough that they ought to know when to retreat from a battle they’re losing, but as I’ve mentioned before, undead creatures are cursed with compulsions that defy common-sense survival impulses. Thus, there’s nothing wrong with your choosing to have them keep fighting until they’re destroyed. However, I think there are a few things that will make them decide a party of PCs is no longer worth the aggravation: being turned by a cleric; being struck by a magical weapon or spell by every member of the party, or by another party member wielding such every time they try to attack a more vulnerable PC; or being struck repeatedly by force, psychic or radiant damage, the three types of damage to which they’re neither immune nor resistant. When a will-o’-wisp decides to retreat, it will simply go invisible, move away and never show itself to the PCs again.
Next: Shambling mounds.