B/X Monsters A to Z: Dryad

 I have a soft spot for non-violent "monsters" and for sylvan creatures, and the dryad, from the Expert Rules, is both of these.

Dryads are described as beautiful female tree spirits. Presumably this means beautiful in the human-like sense, though we don't get any more in the way of description, so, as with many B/X creatures, the specifics are up to the DM. Sometimes they're depicted as very human-like, perhaps with a greenish cast to their skin or hair; other times they've more elven, or have brown, bark-like skin. In any case, forest hues do seem appropriate. Dryads dwell in dense forests and woodlands, and each one lives in its own tree, to which its spirit is bound, such that if she is taken more than 240' from her tree, or the tree dies, the dryad dies. She may also merge with her tree, becoming physically part of it and completely unobservable. It's not stated whether she can observe what goes on around her in this state, but the time-honored trope of characters feeling watched in enchanted woods does make this a reasonable assumption.

Dryads are pretty durable, with an Armor Class of 5 (support for the interpretation of bark-like skin) and 2 Hit Dice, but they have no physical attacks and listed Damage of 0. They're stated to be very shy and non-violent, but also suspicious of strangers. If she feels threatened (which can involve as little as approaching her) a dryad can cast a powerful charm person spell, requiring a saving throw vs. spells at -2. If the spell succeeds, the victim approaches the dryad's tree to be drawn inside; if not rescued before that point, the victim will never be seen again. No limit is mentioned to how frequently this charm power is usable, implying a dryad can use it at will without restriction. Dryads are pretty skittish, with a Morale score of 6, so it won't take too much to get them to retreat, but there's a strong possibility they'll be taking one or more of your allies with them. In any case, they can't pursue a fleeing party very far, even if they were so inclined, which is unlikely in itself.

Exactly what happens to characters drawn into a dryad's tree is not clear, but "never seen again" does seem to imply that there's nothing other party members can do (save perhaps for a well-phrased wish) to recover them. Perhaps the dryads themselves can release them, and as much is outright stated in PC1: Tall Tales of the Wee Folk, but that's a BECMI supplement, so not necessarily B/X canon. I'd imagine that killing the dryad or her tree is a sure way to lose a charmed character beyond almost any hope of recovery, though.

Based on a Number Appearing of 1-6, it seems that dryads, and hence their trees, are often found as close neighbors. When you offend a dryad, it's likely you'll have to deal not only with her, but with a few of her sisters as well. 

The takeaway from all this is that it's unwise to attack, or indeed make even vaguely aggressive advances toward dryads. A TPK is less likely than with many other monsters, but there's a high probability of losing a character or two. It's not a wholly fruitless endeavor, though, for a sufficiently bloodthirsty (sapthirsty?) party, as dryads are noted to keep hoards of Treasure Type D hidden beneath the roots of their trees.

Assuming your party is not the sort to brutally attack and slaughter nonviolent beings, though, what use are dryads to your campaign? They can be valuable sources of information about their forest, its denizens, and events both recent and long past (since they likely live similar lifespans to their soul trees). The challenge for an adventuring party is how to approach without alarming them, which could be at least a mini-quest in itself. They could be guardians of special items or treasures, magical locations such as pools or fairy rings, magical or medicinal plants, or even enchanted wood from their trees for the making of magical weapons or other items, again requiring the party to figure out how to get on their good side. 

Depending on the relationship of humans to nature in your world, dryads might even be hostile by default, the guardians of deep woods where men fear to tread, earning reputations as vengeful witches or forest demons to those who escape their fell enchantments.

All in all, dryads are well-designed creatures, very powerful within their sphere but not overpowered. They're generally benign, but their suspicious nature and Neutral alignment make them prone to shoot first and ask questions later, so a friendly encounter is far from given. They're rife with role-playing potential and world-building flavor possibilities, and well worth incorporating into a classic D&D campaign.