The birch crag appears to be a large withered tree, twisted into the vague tortured shape of a man. Two large foot-like roots hold it upright, while several massive networks of branches form its arms, ending in formidable claws. It glowers at you, sweeping its wooden claws through the air menacingly. This being would tower a few feet above even the tallest Gor Tog.
The crags come in many different types of wood (yew, birch, sanrisi, ironwood, etc), but don't seem to be any different from each other.
When at melee with a crag a writhing root tendril can surface and begin attacking. If you retreat hide or kill the crag then tendril will burrow back underground. When you kill one tendril a new one will surface. The tendril carry no loot or weapons and can not be skinned.
A writhing root tendril bursts up from the ground behind you, lashing about violently!
A writhing root tendril wriggles rapidly back into the earth below.
Crags do not operate under standard spawning mechanics, instead it is a byproduct of a Glistening Scythewing Larvae metamorphosing into a Dusky Scythewing Moths. Once a Moth emerges from its cocoon, the tree it was attached to will turn into a Crag in 230 seconds.