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
As a being made of wood, Sylvans are quite flammable. Their Roots ability will stun two characters and deals nature damage over time; this can be troublesome when facing more than one sylvan.
Sylvans are also perhaps the only creature that will use a power without having line of sight. For example, if a mage casts Blizzard from behind cover and without having line of sight, a Sylvan may retaliate with Roots.