Spirit of the forest
This thing is frightening. A good-hearted experiment gone awfully wrong. In times when the dark druids and their defiling arts devoured most of the great forests of old. In some places the spirits of the forest rised to meet the conquering villains. Monsters like this were born. The birch, elm, hickory and great oak, grown to fuse together in a great treant-shape, strong enough to drain nourishment from rock bottom if needed, that was the spirits' idea when this being was being grown. There were things in the rock that the forest couldn't anticipate, witch-iron from the old kingdoms, abandoned spells, still going strongly in the deep. And when the thing was growing, it devoured one of those items, the item which end up deep into its being and clasped its heart in claws of evil witch-iron. You can barely see the lump that is the being's heart pulsing inside its sparse ribcage. It wasn't the avatar the forest was hoping for, their hopes were high, they even formed a crown of osage and lemonwood to its brows. But they didn't get a king, all they got was a mad monster they had to lock away, far beneath the forest's soil.
It seems to have a powerful aura of prestige. It is an avatar of nature and VERY HUGE of size.