You found elves! number appearing? roll in this table:
1. Average elf (1HD, no armor, short bows, with the bow expertise feat granting an extra 1d6 on attacks) They can climb trees very fast.
2. two elves
3. three elves
4. druid (level=1d6, 1d3 spells, with heal and speak with plants being the most common. Speak with plants is the key to gather Sleep Powder from a specific flower: they carry 1d6-3 uses of it. Blowing onto the powder works as a sleep spell. Possible treasure to be found in elven lairs)
5. veteran hunter (no armor, level 1+1d6)
6. Roll twice. On a repeated result or another 6, raise the level of everyone by one
Elves make their lairs in deep forests, and whenever they are on that terrain, they always have advantage to surprise rolls. When on real combat, they will always try to shoot from the treetops: anyone trying to shoot them from below does it at -4 due to the height difference and concealment. Looking for a hidden elf in a tree works as looking for secret doors (5+ on a d6) with advantage if they make any noise, so is a good idea to trick them into talk even if just to locate their silhouettes.
This means that elves are pretty though encounters for their size. I've actually made them more OP each time I use them, because as a GM I enjoy monsters that are hard to kill but offer plenty space for retreat, negotiation, roleplay and using spells and other dirty tricks. As a side note, I am trying to reserve fair combat for "enraged" or "dumb" monsters like golems, but I hate when the PCs resolve and encounter with civilised humanoids and I find out a posteriori that I made them not use tactics at all.
An elven lair consists in sparse cozy shelters carefully hidden into the treetops, and the rest of the hex and the adjacent six (if suitable) are hunting grounds. Elven life revolves indefectivelly around hunting by means of their short bows, though they also carry daggers carved from horns and bones, which they call "fangs". They can prey on many things appearing on your random monster table for the area; including shit like tiger beetles or giant scolopendras. I don't imagine owlbears being edible but they are probably prized for their skins. But of course, it is a great idea to make space for the antelope and the boar, always present on b/x bestiary. Elves cook their food and make campfires to do so if they believe they won't attract too much attention (this is a good example of a good surprise roll against elves).
As opposed to Trow, whom are more proclive to hunt big game par force (that is, succintly, using dogs and coordinate tactics to pursue a catch until its tired and presents battle) elves use stealth. Trow use hunting parties as a social event, and a way to introduce young boys to the customs of riding and battle (I'd love to write an entry about that specific topic soon, cross fingers), while their elven counterparts often hunt alone, and rarely in groups bigger than three. They never use dogs (and rarely have pets that cannot fly or at least climb).
It's not just that they are hard to detect: Elves hardly ever talk, not even amongst them. When PCs arrive into elven territory, they might receive a silent warning in the shape of arrows shot in the ground before them, with the shooters well hidden in the foliage above. If that doesn't work (as always happens with PCs) they might shot a sack that can be pierced, or try to wound someone, to make the whole party turn their back (that's like their neutral reaction roll on their territory). They don't want their sacred groves profaned with bonfires and their buck deers flushed by the noisy trow. Or they are just appalled by the idea of two marching kobolds breaking the peace with constant jokes and laughters.
They do not understand why other races talk so much, like there was so many things to be said. Though they might seem serious and stupid, they do have sense of humor and keep normal relationships, but they somehow manage to do it talking much less and using a lot of implied acts, which are amplified by their surrounding lack of speech.
If required to, they will try to use a single word; two in case of need. Four consecutive words or more are considered a poem for them (and they do write four word poems. This is not an exageration); but they never use verbs in any form. For some metagamist reason, they are unable to use them or appreciate them and so, their speech sounds always precarious to trow and kobolds. 
Elves are plant-like in apparence, and dress in pelts; as heavy and hooded as the climate requires. Their color is a greyish green and their hair looks like leafs. This has no more biological effects beyond that, though poison seems to affect them less virulently (bonus in saving throws vs poison) And, of course, there is the fact that a tree will grow from their corpse once they die. This trees are sacred and elves will protect them violently, with at most an awkward explanation. This tree is believed to hold the soul of the deceased for an indefinite time after his death, and the tribe's druids can partially communicate with them. Many times, elves who are about to die decide to wait for death right outside the limits of their forest; knowing that this will cause a subtle inertia for new groves to expand towards their gravetree.
Though elves are individualistic in nature, when a decision involves the whole tribe they tend to value the words of the elders: A veteran hunter that is now a potter by the river, or a venerable druid (clasically a woman). In addition, the opinion or the blessing of the departed elders, now in tree form, is also sought through the druid's abilities.
A silent elf sitting on a branch could be doing anything from sleeping, contemplating nature, listening for prey, thinking or practising meditation, all in the same unassuming position. By doing the latter, they believe that they weaken their illusory miconceptions and are innately guided to their place in the nature cycle, like the other animals do.
Elves are to be a playable race (though not used them as such yet). I've used ambiguous mechanical terms in this entry, but applied to my Trow Fortress rules, their dexterity must be high, so I will make it so they must allocate their best score onto it, and instead of getting regular HD increases as fighter, they can at certain levels increase their dexterity or magic by +1, no roll required. This will help them get into their presumed roles as veterans. As a side effect, some elves can max out dexterity and become elven monks (expertise: combat without weapons) and even have some magic buffs, related or unrelated to combat. But that is something to be written another day. I hope you liked my take on indian-dryads.
