Have you seen the Jack-in-the-Green?

A mischievous or malevolent fairy of the woods, the Jack-in-the-Green can be a hazard to travellers. Occasionally, however, the creature will prove helpful in return for assistance towards some obscure goal of its own - the retrieval of a treasure from holy ground, perhaps, or the driving off of a monster that has moved into the forest.

Jacks-in-the-Green appear as ugly sprites with skin the colour of moss. They dress entirely in greens and browns. A few wear mail shirts of corroded bronze, doubtless looted from ancient barrows. They
cannot abide the touch of iron, but arm themselves with bows, cudgels and daggers of bronze or flint.

 Typically encountered in groups of 1 to 12, Jacks-in-the-Green have an equal chance of being hostile or 'friendly'. If hostile, they will act as follows:

D6
1. Remain unseen but direct the travellers towards an area of danger (a swamp, crumbling ruin or monster's lair, for example) through enticing words ("There's treasure that way.") or warnings ("Don't go that way - that's where the spiders are.")  from the undergrowth.
2. Pelt passers-by with stones and twigs.
3. Drive forest beasts into the travellers' path (1D6: 1. birds 2. deer 3. boar. 4. wolves 5. bear 6. monster)
4. Shoot arrows at the travellers from behind trees.
5. Block the travellers' way and demand they turn back.
6. Launch a full ambush.

Alternatively, these tactics can escalate from 1 to 6.

If friendly, they will offer one of the following in return for a favour:

D6
1. Guidance to any part of the forest that the PCs wish to visit.
2. A bag of gold: 3d20 ancient gold pieces larger and more valuable than contemporary coins (worth double). These coins will last for three days after acquisition, at which point they will turn to leaves, mud and earth. Those paid with the coins may be annoyed (and vengeful).
3. A sealed jug of fairy wine. Made from brambles, this is delicious, addictive and extraordinarily potent. An hour after drinking it, those who fail a saving throw will be unconscious; those who make it will be confused, incoherent and very happy. Perfect for bribing guards ...
4. An wolfskin that gives a +5 bonus to any stealth-related roll. It also conveys the curse of lycanthropy at full moon; this is permanent until the hide is burnt.
5. A homunculus made of woven twigs and leaves. Though apparently inanimate, it will act as a servant and guard for D6 weeks, before disintegrating into its constituent parts.
6. A small blue egg containing the life essence of an ogre who lives nearby in the forest. He will begrudgingly obey the owner of the egg in all matters (though will look to murder them in their sleep to retrieve the egg, which he will then swallow).

The favour the Jacks-in-the-Green request could be one of these:

D6
1. The retrieval of a small wooden idol from a ruined and overgrown temple (the Jacks cannot enter holy ground).
2. The slaughter of a group of humanoids who have set up noisy and rambunctious camp in one of the  Jacks' favourite groves. These humanoids are heavily armed and armoured with iron or steel weapons, so the Jacks would rather have someone else do their fighting.
3. The removal of a dangerous creature from a forest river.
4. Obtaining of the skull of an armoured warrior who lies in the undergrowth. His visored helm, affixed to the rest of his armour, prevents the Jacks from doing so themselves.
5. Burning the hut of a local witch. For reasons of their own, the Jacks' animus is against the building rather than its occupant.
6. Retrieving a great blue gem from the belly of an ancient serpent that lives in a cave deep in the woods. The gem is very valuable, but failing to deliver it to the Jacks-in-the-Green will earn their undying enmity.

Refusing to strike a deal with a 'friendly' Jack-in-the-Green turns it hostile, typically resulting in escalating aggressions as detailed above.

In Whitehack terms, a typical Jack-in-the-Green has AV 11 (13 with missiles), HD 1 and AC 2 (agility rather than armour). A few wear ancient bronze hauberks and helmets (AC 4), and some carry tomb-looted shields. Many have bows or darts, and all are adept at throwing stones.

Some are extremely small, with a single hit point and a single point of damage for attacks. Occasionally, some are larger - 2HD, AV12 (14 with missiles), damage by weapon.

All Jacks-in-the-Green can cast spells of a petty sort. Each can cast one per encounter, at no cost to its HP. The available spells are as follows:

Snake-arrow: if a Jack misses with a shot or thrown stone, he can instantly turn the missile into an aggressive and venomous snake (AC2, HP 1, AV 10, damage 1 + save vs poison).

Wasps: a swarm of stinging insects attacks the target. Damage is insignificant (though painful), but an affected character will be too busy beating the bees off to do much else. A victim of the spell counts as being in combat with the bees, so that any other opponent gets advantage (best of two dice rolls) if they attack. Bee victims are disadvantaged in attempting to fight back, although they may use a lit torch or other fiery implement to attack the bees (AC6, HP6). The bees disperse in D3 + 1 rounds, or if their creator is killed.

Roots: the target must make a DEX check or be gripped around the ankles by surprisingly strong plant forms that sprout from the earth. A sharp implement and a DEX check with disadvantage are required to break free.

Greening: if a saving throw is failed, the target's skin and hair will be turned green. The effect is permanent unless the caster is killed or persuaded to lift the spell (perhaps by performing one of the favours listed above). A Jack-in-the-Green may use this spell as a preliminary negotiating tactic (casting the spell before he introduces himself, of course).

Lurker: something huge appears to be coming through the forest, to appear in D6 rounds. There is crashing, stomping and a terrible mournful call - but the noises end just as the illusory beast should arrive. Characters who investigate before it does will see branches moving and the undergrowth shifting, but without visible or tangible cause.

Startle: a great terror comes upon any animals associated with the party (horses, mules, dogs, etc.). These will bolt into the forest unless the PCs spend three rounds per animal calming them. Any halters, etc., will be unravel themselves from branches.

A Jack-in-the-Green's lair is typically an old barrow in a forest glade. Several of the creatures will typically inhabit such a mound. If followed in, however, they are seldom to be found within; there are old roads in the dark places of the earth that mortals cannot follow. Ancient grave goods may be found, however - the bronze swords and knives that the Jacks-in-the-Green wield, torcs and arm-rings.

Some say that a Jack-in-the-Green is the spirit of a child lost in the forest or stolen by fairies. When a Jack is slain, its body dissolves into a heap of twigs, moss and leaves. On occasion, a tiny skeleton may be found within.