To that end, the other day I saw some pretty cool banyan trees, aerial prop roots hanging down from above, stretching for the ground, ready to create more and more trunks in an ultimate banyan grove all centered around a single huge trunk of intertwined trunks. Some quick Wikipedia research informed me that banyans are sometimes called "strangler figs" for the way that they grow around a victim tree and "strangle" it with their roots; and further that, as a fig tree, they are home to fig wasps, which developed symbiotically and cospecialized with fig trees, the fig providing the wasp a place to lay eggs and mate, the wasp pollinating the fig.
Hence the double feature of Strangler Figs and Fig Wasps ... Today, the Strangler Fig:
or Banyan Tree ...
Called a "strangler fig" because it is a parasitic organism that "strangles" its host--the banyan tree actually grows atop another tree, its seed having landed somewhere in the host's upper branches, and grown like a vegetative octopus from there. Early on, the banyan actually looks like tree roots growing down around the trunk of the host tree; later, as the banyan swallows or "strangles" the host and sends out limbs of its own, these down-growing roots actually become almost trunks themselves, creating the illusion of a small grove where the reality is one large down-growing organism (as opposed to an aspen grove, all one organism, but growing upwards).
After enough time, the banyan actually kills its host (after it has grown large enough to sustain itself), and as the host tree decays within the banyan's central trunk, it leaves behind a hollow space, perfect for any variety of spirits, jungle-creatures, or bandits to dwell within or inside which to store their valuables ...
And so, to D&D-ize the banyan tree ...
No. Appearing 1 (1-6)
THAC0 per HD, e.g. 12-10
attacks: suggestion or 7-12 roots
damage: 2-7 or charm
A strangler fig large enough to attack a party of humanoids has grown into a veritable grove of its own, with prop-roots resembling entire trees and aerial roots hanging down from the canopy throughout the "grove"--hence its hit dice of 8 to 12; feel free to shrink or expand the number according to the age/size of the "grove".
The grove feels to be a place of peace, which is the will of the fig attempting to lull prey into a sense of security. There is a 1 in 3 chance that from 1-4 animals of various kinds (e.g. monkeys, lions, agouti, etc. etc.) are resting beneath the banyan's limbs, subtly tapped by its roots, charmed, and succumbing to its appetite. These animals may be used by the banyan to attack intruders if the intruders are hostile.
Suggestion: All those who enter the grove (i.e. pass into the shadow beneath its canopy) must save against spells (or a make Will/WIS save), or they find the grove to be a perfect place to make camp, and make an argument for settling down for the day, or at least for returning before day's end ("Look at how peaceful the animals are here--lions and monkeys lying down together!").
The banyan being a slow-moving tree, creatures must spend at least 10 minutes under its canopy before its roots can act. At that point, those beneath must make a save against paralysis or be tapped by an aerial root. After that, 7-12 roots may act each turn, either directly attacking characters or charming them.
A root that attempts to charm a person descends and rests on their body, attempting to "tap" into them; the character must make a save against paralysis (fortitude or reflex? constitution or dexterity? choose what you want in newer editions). Characters who fail are "tapped" and cannot leave the vicinity of the grove, and can be compelled to fight against hostile intruders. Over the course of the root's accelerated growth, they become part of the banyan in 1-6 days, entombed within a new root-trunk. A cure disease spell will dissolve the root's connections inside the body, though a character thus freed will suffer 3-18 damage from separation.
Otherwise, the roots attack with slams like treant-limbs, dealing 2-7 damage per blow.
Any treasure is to be found within the main "trunk" (which may be hollow), or embedded in the down-growing aerial roots, in either case being the treasure of those entombed within--which will, of course, entail 10 minutes or more work to extract ...
Parasitic though they be, banyans are also often symbiotic homes to other creatures as well.
One half of banyan trees are home to a swarm of Fig Wasps, insects that co-developed with fig trees in a relationship even closer than that of the normal process of bees pollinating flowers.
One in three banyan trees is also home to a dryad, a bisan (from Oriental Adventures), or some other tree spirit; these are (1-2) the spirit of the host tree, desperate for help destroying the banyan, (3-5) the spirit of the banyan or some other spirit dwelling within the hollow of the "trunk" and which will rise to its defense, or (6) an unaffiliated spirit that dwells within the hollow trunk or spreading branches, but which is itself independent of the banyan.
Further relationships are eminently possible:
Consider an entire human keep, made of stone, swallowed by a vast banyan and now home the ghosts of its former inhabitants!
OR, a peaceful banyan, converted to (Buddhism) by a saint in expectation of better future lives and escape from the wheel of suffering, which holds under its branches/roots a permanently peaceful gathering of merchants and wanderers, guarded from evil by the power of the tree.
(the etymology of "banyan", according to Wikipedia, is that "banyan" meant "merchant" in Gunjurati Indian; and such merchants would spread their wares beneath the shade of certain [banyan] trees)