There are many methods that the Dembraava Wilds seeks to kill the humans who enter it, and the Dendrognaths are one of the most potent. Called "knock jaws" by everyone who isn't a wizard, dendrognaths plague the entire forest. It is not a tree. Nor is it an animal. It is a trap set to murder those who anyone who passes near it.
Although the process has never been observed, it is believed that dendrognaths are created when a Deathwalker decapitates a wild animal, usually a wolf, heron, or snake, and splices the head on a recently cut section of living tree. Any kind of tree will do, although larger trees are preferred. Through some unholy process, the head embeds itself in the tree and begins sapping it. The bones in the skull are quickly replaced by wood, and the head will grow larger as long as the tree does. The tree will never grow an inch--but the decapitated head will swell ever larger. There does not seem to be any upper limit to how large the dendrognath may grow, and some truly titantic specimens have been recorded.
While the tree seems healthy enough (although perhaps a bit stunted), the animal head appears mouldering, with milky eyes and a black tongue. The head does not move in any way until it attacks. Part of their horror comes from their appearance. A ranger might brush aside a branch, climb down to a stream bed, and suddenly realize that he is standing only a few feet away from a ragged wolf head, black lips pulled back in a terrific, snarling rictus, the entire thing four feet long. The ranger hesitates--the milky eyes of the thing don't move, and maybe it hasn't noticed him, or it's asleep. . . but after that one half-second has elapsed, and hope has fluttered up and died, the thing bursts from the trunk in a splinter of rotten wood and torn flesh, slams into the man's chest, and summarily rips his throat out.
Although human mages and necromancers have been unable to duplicate the effect, it does not seem to be difficult to create; The Deathwalkers have constructed thousands of them. And in the darkest places of the forest, where no ranger's foot has trod in decades, these terrible jaws grow the largest and most fearsome. Similarly, where the trees are the oldest and the tallest, the Dendrognaths grow the fastest, for there is more power to be sucked from a mighty oak than a sapling. And while dendrognaths can be found throughout the forest, they are 'seeded' at locations of tactical significance: safehouses, sources of fresh water, and the protective totems that stop the forest from spreading. There are even rumors of a hidden glade deep in the forest, ringed by dozens of towering oaks, each with dozens of eldest lupine dendrognaths leering from their trunks. And in the center of the glade, stairs downward.
Some insist that animal heads, even animated ones, have no means of jumping, let alone locomotion. When this is asked of a ranger, the woodsman's face will darken, and the response will invariably be something along the lines of, "Well, that hasn't bloody well stopped them, has it?"
Each dendrognath attacks only once. Although it has no mind, it is filled with the same spirit--that same hatred--that fills the Mordanfey and the Deathwalkers. When it senses an outsider, the head is capable of leaping great distances towards its prey. Once it catches you, it will tear open your belly, or impale your eyes, or sink its fangs into your lungs. Once the attack is completed, successful or not, the head de-animates and will rot normally, except for the wooden 'skull'. Which are, of course, highly prized by the right parties. Wizards can always be counted on to be dumb enough to buy evil, cursed doodads. And in Basharna, the 'skulls' are painted and hung in wardog kennels, in the hopes that the savage spirit will enter (or at least influence) the packs of fighting dogs.
I'm not sure why we haven't seen dendrognaths derived from human heads. The Deathwalkers certainly have enough of them. Then again, if you ever find Colton by the fire after he's crawled inside a bottle of whiskey, he'll tell you otherwise. He's been further in the Dembraava than most men can dream. That's probably why he drinks so much.