BX Monsters A to Z: Harpy

 Here's a monster that seems to be the product of a mashup between two very similar mythical creatures: the harpy.

Mythological harpies are hybrids of women and birds who were traditionally associated with storm winds. Some early incarnations describe their human aspects as attractive, but mostly they were held to be quite repugnant. So far, it matches pretty closely with the D&D harpy. What it's missing is a song that can charm humans. That ability is traditionally associated with another mythical being, the siren. While sirens in later years are usually thought of as mermaid-like creatures with the lower bodies of fish, in their earlier incarnations they were described as having the faces of women, and the bodies, feet, and wings of birds. The D&D harpy is described more specifically than either, having the lower body of a giant eagle and the upper body and head of a hideous woman. 

Stats-wise, harpies are pretty formidable opponents for Basic-level parties. Their AC of 7 isn't impressive, but they have 3 Hit Dice and attack three times per round with their eagle talons and a weapon for 1-4/1-4/1-6 damage. That's a maximum total of 14 and an average of 8.5 if all three attacks succeed; a hefty amount for PCs who aren't likely to be far from single digits in their own hp. If you can manage to ground them somehow, harpies waddle along at a sluggish 60' (20'), but otherwise they're flying at 150' (50'). Number appearing is 1-6 in the dungeon and 2-8 in the wilderness, which is frankly a killer encounter to Basic-level characters and a pretty stiff challenge for low-to-mid-Expert ones as well. Their morale is a bit shaky at 7, so if you get some good licks in early, you might be able to drive them off. It is also stated that harpies are resistant to magic and make all their saving throws at +2 (not just saves against magical effects, apparently). 

Despite being able to deal some respectable damage, the real danger of an encounter with harpies is their song, which acts as a charm person spell. Rather than a spell cast at a single individual at a time, the description seems to imply that the song of a group of harpies affects all within range of hearing, regardless of the numbers of either the harpies or the party. Anyone who makes their saving throw vs. spells is thereafter immune to further singing by that entire group of harpies for the rest of the encounter. Those who succumb to the charm approach the harpies, resisting attempts to stop them but otherwise making no attacks. Harpies apparently do not use charmed victims as slaves or guards; their charm is entirely for the purpose of luring people to them to be slain and devoured. The standard spell states that charmed persons will refuse commands to kill themselves, which would presumably include commands to stand idle while their charmer slaughters them as well. It isn't explicitly stated, but given the very limited purpose of the harpy's charm, it's likely that this caveat doesn't apply to them: charmed characters will passively allow themselves to be killed.

Given the mass charm effect of harpies' songs, getting the first action in the encounter, either by surprise or by winning initiative, is even more pivotal than is typical in other encounters. If the party goes first, a silence 15' radius spell can neutralize the monsters' greatest advantage, and getting in some solid damage might force an early morale check. Should the harpies act first, it's very likely that the fight will be between the entire group of harpies and half or fewer of the party's members, i.e. those who make their saving throws. Access to a dispel magic spell or effect (in the hands of someone who makes their save) could be an absolute lifesaver.   

It isn't explicitly stated, but a generous DM might, if the players are forewarned of an encounter with harpies, allow the method made famous by Odysseus against sirens of blocking ears with wax, to negate the effects of harpy song. If so, it would also be a fair twist to disallow any verbal communication between characters while such measures are employed. 

A harpy lair contains a Treasure Type C hoard, which is hardly worth the danger of confronting the monsters. 

It is unknown if harpies ever have non-nutritional relationships with other creatures, but if the DM decides they do, there are all sorts of possibilities for magically coerced and mutually beneficial arrangements between them and all manner of other monsters and NPCs. A well-fed flock of harpies might serve as kidnappers for a dragon or an evil wizard, restraining their appetites to instead carry their charmed victims back to the nefarious clutches of their associates. Alternatively, a flock might employ charmed humanoid monsters as guards, or use them as bait to lure in charm-resistant prey.