Treant

A treant on the party's side is a forest deciding to walk with them, briefly, on a matter the forest cares about. Treants do not adventure. They listen for a long time, agree slowly, and then commit completely to one specific errand. The right setup is a logging operation threatening their grove, a fire cult marching through old growth, an undead infestation rotting roots, or a druid PC who has spent in-game weeks earning trust.

In the field, the treant is artillery and a wall in the same body. Hail of Bark throws 4d10+6 piercing at +10 to hit out to 180 feet, which means the treant outranges almost anything the party fights and contributes serious damage from the back of the marching order. Multiattack on the ground is two Slams at +10 for 16 bludgeoning each, with reach 5 ft. and Huge size, so it covers a lane the party cannot easily flank around.

Animate Trees (1/Day) is the table moment. The treant animates up to two trees within 60 feet, each running its full stat block (138 HP, AC 16) for the duration. That is the difference between a hard fight and a rout, so use it the round things are about to go badly: the cleric drops, a hill giant Multiattacks the rogue, an enemy spellcaster drops a wall. Three treants on the field do not lose to most CR 9 encounters. Save it; do not open with it.

The treant's exit is the moment that defines the alliance. Once the threat to the wood is gone, the treant goes back to standing exactly where it started and stops talking. Druidic, Sylvan, Common, Elvish: the treant speaks all four, but slowly, and only when it has something worth saying. The party should feel like they helped a forest, not hired a siege engine.

Have the treant ask the party for a single small thing in return: bury this acorn in the ash field, carry this seed three days east. The favor outlasts the fight.

A hostile treant is usually a treant that has decided the party is the threat. Maybe a ranger PC has been carrying a torch through the deep wood. Maybe the bard's spellbook is bound in elm. Maybe a previous party clear-cut a grove and word travels through roots. Sell the misunderstanding for a scene if you can; if the players cannot talk it down, the woods themselves are now the encounter.

Open at range. Hail of Bark is the play: +10 to hit, 28 piercing average, 180-foot range. Stand inside the treeline and pelt the marching column before they can identify which trunk is moving. After two rounds of stand-off, the treant closes for Multiattack: two Slams at +10 for 16 bludgeoning each. Reach 5 ft. and the Huge footprint mean the treant does not need to chase; it picks a lane and the party either holds it or dies in it.

Fire is the obvious answer and the treant knows it. Vulnerable to Fire is a real problem (double damage), so the treant should be aware of any open flame and use Hail of Bark to pulp the torch-bearer first. Resistance to Bludgeoning and Piercing wastes most martial damage; the party that brought a flaming weapon, an alchemist's fire, or a Burning Hands gets to make the fight short. The party that did not is in a very long fight.

Animate Trees is the panic button on a 1/Day. Use it when the party has just dropped the treant's HP into the lower half. Two more treants stand up out of the underbrush, each with 138 HP and the same Slam, and the entire encounter resets. If even that goes wrong, the treant retreats by walking into the densest part of the wood; with Siege Monster doubling damage to objects, it can break a path through stone walls or palisades to escape into terrain the party cannot follow.

Describe the leaves shaking on the trees the party walks past, three rounds before initiative. The wood notices them long before the wood acts.