Trefolc

Tell me, Magnus. Why do you think humanity has such an instinctual fear of the deep forest? Do you think it to be a simple fear of the dark, or fear of the Pixies? Of course not. It is because we fear the wrath of nature's prized guardians--Trefolc are not to be underestimated. But fear not, don't harm the ecosystem they defend, and they'll leave you alone.

— Merlin, Archwizard of Camelot

Trefolc are a unique species of creatures that defy all normal logic, holding little to no connection to actual evolution or natural biology. They are creatures made up entirely of various forms of plantlife meshed together. Some appear to be a single large tree that has morphed into a shape of a humanoid or other animal, others appear rooted to the ground and dominate the plantlife around them instead. The variety is widespread, and so most have begun naming the Trefolk based not on their appearance, but on their habits and materials.
All Trefolc are spirits that Nature itself has taken an interest in, and offered its hand to them after death. Providing them with new bodies crafted from nature itself and allowing them to possess plant life as if it were their own flesh. These spirits are then charged with protecting various locations across the world based on their power and personality. Due to this, Trefolc are nearly impossible to "kill". But they can be destroyed or driven back through pain. Usually threat of more damage to their domain is even more effective than directly harming the Trefolc's body.
The most learned of scholars will know that a Trefolc's life force is granted to it through Nature, specifically through the land that it is charged with protecting. Should that land be destroyed, Nature will view this as failing its oath. As such it will lose its blessing and become nothing but a normal spirit once more. As such even when enraged, Trefolc can often be talked down if there is a threat overhanging the land it is guarding. The individual personality of a Trefolc varies entirely from one to the next, and as such, there is no predicting if diplomacy without this threat will be viable from one Trefolc to the next. If there is no other option then the use of electricity and cold is recommended against Trefolc, as flame against living plants is ineffecient.
On average Trefolc will not harm those that enter their domain unless they show themselves as a threat against the balance of nature. For instance, hunting for a meal while within the forest is seen as natural. But hunting dozens of beasts at once from a small forest could lead to a complete shift in the natural order, and is therefore viewed far more aggressively. Just as chopping down a single tree would be forgiven, but a sudden swath being cut through the forest would enrage a Trefolc without a doubt.
Trefolc can even be communicated with, and will often enjoy news and information from the outside world. This is due to their previous lives influencing their current duties as spirits of Nature. It's not uncommon to hear tales of wanderers that spent a few days admiring a forest, or speaking to the trees fondly only to be protected as long as they traveled its lands. This joy in finding those that respect them, and their forest enough to talk to them is a core aspect of most Trefolc's personalities.
Trefolc enjoy wandering through their lands, and simply talking with their plants. Speaking in the slow language of Draesil to the various plant life around them. Passing their days peacefully while tending to their lands to the best of their ability.

Ent

Whenever you see a Trefolk that has been grown from wood and trees, we call them an 'Ent'. Named such due to the first sightings being monstrous trees, they were mistaken for Giant-kin, you see. We thought they were a new sub-species. Foolish in hindsight, but the name has stuck.

— Atennus Tresmenne in "Species Known and Unknown"

'Ent' is the term used to describe any Trefolc that has possessed a singular tree of any size or sort, even some large bushes at times. But the requirement of the term is their base form being made of solid wood. Beyond this, the specifications begin to grow incredibly vague. Often they will be found guarding dense forests or jungles, protecting their lands.
It's not unusual at all for an Ent to be the size of a small sapling or great oak. There is little defining this aside from simple individual preferences. Depending on the species of tree that they overtake, an Ent could be incredibly strong or focused on agility to traverse through the forest itself. It isn't unusual at all for an Ent to call upon the trees surrounding it to attack their enemies or to open up and help guide kind travelers through the forest as quickly as possible.

Ent by Brian Valeza

There have even been cases of Ent's being possessed Gleam-Fruit Tree's and making contact with the Dila'Mori to act as scouts along the rivers. With this immense variation its difficult to know the best methods of dealing with them, but due to their basis a creatures of wood, the usage of heavy chopping or crushing weapons are usually prefered. Even more prefered than that is offering the Ent nuts or fruits from trees of other lands as peace offerings. This will often placate an Ent, and may even earn its favor.

Nymph by Ket-DawnAtSunset

Nymph

If an Ent is the spirit of a direct, firm person that would rather strike you to settle an argument; then a Nymph is a crafty witch of the wilds. Offering her friends great gifts, food, knowledge, information, and protection. Yet, should they be betrayed they will quietly roll over. Then, five years down the line, you will sit within the center of your house burned to the ground next to the bodies of your family, wondering 'how'? Patient. Cunning, Ruthless. Powerful allies, or terrifying enemies. That is a Nymph.

— Magnus Everlight, Grand Scholar of Conlis

When a spirit takes over a plant of some sort that isn't a tree, they are referred to as a 'Nymph'. They often will take over large briar patches, flower fields, abandoned gardens, vines, and similar plant life. In doing so they are far less battle-ready than the powerful Ents or crafty, yet deadly Swamm. Nymphs tend to prefer keeping to themselves and offering peace instead of battle. They are the most likely to take on physical forms that are viewed as "attractive" and "humanoid" as they can be with their plant-based forms.

Often preferring to communicate with travelers, feed them, share blessings, and let it be known they would rather make love than war. It's not unusual at all for a small village to worship a Nymph, offering the forest protection from outsiders in return for the spirit helping to feed the village through harsh winters and the like. This said, it isn't as though a Nymph is to be underestimated, as even if they don't enjoy direct conflict they are still Trefolc. Their duty to protect their lands, and perhaps even more so, their friends--is a deadly thing to toy with.
Nymphs are often unafraid to abuse toxic plants, briars, pitfalls, and calling upon their many allies, often Pixies to defend their homes. Hallucinogenic gasses, toxic flowers that cause the body to shut down, nightmare-inducing scents, briars, and animal life hunting you at every turn. All the while briars, and vines attacking and hindering you at every step while insects try to invade your orifices and shred through your flesh. This is the wrath of a Nymph, and due to that many prefer the idea of fighting the Ents by comparison.

Swamm

Ent are forward and powerful. Nymph are kind yet ruthless. But the Swamm are simple spirits, they simply wish to be left alone. Crafted from hermits that wish to only be stagnant, calm, unmoving. Focused on their roles as guardians they don't usually bother the few that come through their moldy tunnels, dark caves or swamps. But, for the few that do manage to anger a Swamm, they should be prepared to suffer a quick, but agonizing death.

— Atennus Tresmenne in "Species Known and Unknown"

Swamm by Maxim Verehim

Swamm are small and fragile compared to other Trefolc. Being made from various types of condensed fungus and mold. This makes their physical states incredibly fragile, but in return, they tend to be perfectly camouflaged for their territories. Worse are the deadly toxins that pump through their bodies, and the sick, twisted fungal spores that they manipulate and control at will. Swamm will often completely ignore intruders into their land unless they begin causing damage the environment itself cannot handle.
This is due to how deadly their territories tend to be, with vetid swamps, dark spirits from those that died vainly within the brackish water full of giant reptiles, undead, rot, decay, poisons, and disease. But in the rare instances that these creatures must bring forth their own powers, Swamm will aim to kill their targets as quickly and painfully as possible for interrupting them. They will cover them in fungus that causes necrosis and take root in their flesh, their eyes, and mouth. Paralyze them so they drown in the bogs or numerous other dark tactics.
It's not unheard of at all for someone to manage to barely survive an encounter with a Swamm, before dying not long after exiting the swamp due to the fungus growing inside their stomach and intestines. Leading to the body turning to fertilizer, and spreading those same spores even further, now outside the swamp leading to the deaths of hundreds more.

Alchemical Reagents

As always with many supernatural creatures, reagents gathered from the bodies of and lands guarded by Trefolc tend to hold powerful Alchemical effects. This is primarily based on whatever the materials were before the spirit inhabited them causing the mutations and sudden influx of energy into them. But commonly sought after reagents are the sap or bark of an Ent which tends to provide enhanced strength and endurance effects. They are also key components to the popular, if rare, "Barkskin Potion".
Nymphs are often sought after for their nectar stemming from the flowers that bloom from their bodies, as well as ground briars and the like. Depending on the species of flower or plant the Nymph is made up of, they can be used for powerful effects to heal and restore energy. Should they take up the bodies of poisonous flowers and plants this can change completely instead to toxins of various effects instead.
Finally, Swamm, due to their rarity are a sub-species that are the least well known. However, the fungus and mushroom that can grow from their bodies and makes up their material is well known for generally becoming the most potent toxins in the world. Causing necrosis, and worse, spreading the spores of the fungus that made up the original Swamm that it came from. Many a story stem from a greedy alchemist that's experiment went wrong, leading to a fungal pandemic dooming a town to a terrible, slow death.
Lands protected by a Trefolc are also commonly mystically enhanced, such as turning jungles and forests to Ironwood, or even causing direct evolutions such as creating Gleam-Fruit Tree's or other variations.