Occasionally, demons attempt to escape the Fade by possessing something other than mortal flesh. by corrupting a tree, for instance, a demon avoids the madness that results from possessing a sentient creature. Demon-possessed trees are known as wild sylvans. On rare occasions, demons possess trees almost completely destroyed by fire but that still retain a spark of life. This union often rekindles the fire that first damaged the tree, resulting in a sylvan that burns continuously without being destroyed.
—From Codex entry: Charred Sylvan