Dryad

A dryad on the party's side is a quest-giver who lives in a specific tree, not a hireling who travels. Her help is real, but it stops at the edge of her grove. Frame the relationship around a problem only she has noticed, usually a logging crew, a poacher, or something rotten in the soil under an oak two hills over. The party agrees to handle it, the dryad opens a path no one else can find, and the favor cycle begins.

In her grove, she is genuinely dangerous despite the 22 HP. Tree Stride as a bonus action between any two Large or bigger trees within 60 feet means she is never where the party last saw her. Open with Pass without Trace before the encounter so the party gets advantage on Stealth and a +10 bonus, then Entangle on the largest cluster of enemies (DC 14 Strength save or Restrained, plus difficult terrain). Once combat is joined, her Multiattack is one Vine Lash at +6 to hit (8 slashing, 10-foot reach) or one Thorn Burst at +6 to hit (7 piercing, 60-foot range), plus a free Charm Monster cast on a target that fails a DC 14 Wisdom save. Use the charm to pull an enemy out of the fight, not to deal damage.

The hard line is the grove. The dryad will not leave it. If the party asks her to come escort them through the next valley, she politely refuses and offers Druidcraft, an Animal Friendship on something useful, or a passphrase the local foxes will honor. Speak with Beasts and Plants is her real strategic value: she has been listening to every tree on the ridge for a hundred years and she will tell the party which ravine the bandits have a camp in if they ask the right way.

A trick worth doing: when the party returns to the grove later in the campaign, have them notice the dryad's tree has grown a foot taller, or sprouted a marking from something the party did. Her continuity is the grove's continuity.

A hostile dryad is a defensive encounter, not a hunt. She does not chase, she does not ambush travelers on the road, she defends a single tree against people who are already standing too close to it. Set the scene by giving the party reason to be there: a sacred spring on the other side of her tree, a logger's apprentice they were sent to find, a relic an old druid hid in the roots. The dryad treats every step closer to her tree as an act of war.

She fights from the canopy. Bonus action Tree Stride teleports her 60 feet between any two Large or bigger trees, so the party will never get a second melee hit on her in the same place. Open with Entangle (1/Day, DC 14 Strength save or Restrained, 20-foot square of writhing roots) on the front line, then Pass without Trace on herself if more woods are nearby. From cover, Thorn Burst at +6 for 7 piercing across 60 feet picks off the back rank one shot per round, and the Multiattack rider is a free Charm Monster against the strongest-looking PC. DC 14 Wisdom save. A charmed barbarian who decides the dryad is "actually friendly" can spend several rounds breaking the wizard's nose for her.

Magic Resistance gives advantage on saves against spells, so the bard's Hold Person and the cleric's Banishment are bad bets. Burst damage works. The party's job is to find the dryad in the brush, hit her hard, and finish before she resets position.

If her tree is destroyed, the dryad dies with it. The party will figure this out by round three, and that is the moment the encounter becomes a moral one. Have the dryad scream when an axe touches the bark, and let the players decide whether they're still the heroes.