Hi, I picked this book up a couple of decades ago at someone's house, liked it and immediately forgot the title and author. I recall in the story that wizards could summon apparitions of demons, or that demons could be summoned into small teams of soldiers prior to battle where they would then fight as one coordinated entity. Early in the book the apprentice tries to commune with trees by standing out in all weathers as a tree. That is all I can remember, can anyone help identify this book?