For demons crossing over into our world, mankind is not always the preferred prey. Possessing humans means risking encounters with powerful mages and templars, as well as other complications. Some demons find it far easier to seek out animals or even plants, assuming that these will make as suitable a host as a human. Those that possess trees are known as wild sylvans.

Generally, only demons of rage, the weakest of the demon hierarchy, will become a sylvan. Once they do, they must spend a great deal of time twisting and molding the host in order to make it mobile, and once they have the sylvan is a powerful and deadly opponent. Other, more intelligent, spirits have also been known to become sylvans, and are generally much less violent, but these are rare.

Slow but immensely powerful, wild sylvans prefer to lay in ambush, waiting for a victim to become lost, tired, or trapped before closing in for the kill. They hide among regular trees, nearly undetectable until they begin to move and to reach. When they do "come to life" as some travelers say, they stand tall, roots forming into legs and feet and branches stretching out into lashing arms.

When not presented with a living target, however, it has been noted that sylvans often fall into a form of dormancy, perhaps brought on by the nature of their tree host. While mobile, they normally return to wherever they were rooted once their prey has been killed. For both these reasons, a forest that has sylvans within can become incredibly dangerous to pass through for very long periods of time.
—From Codex entry: Wild Sylvan