Locust

They were locked away in the Earth. It is impossible to say how many there were. No manner of reckoning their number had yet been invented when they sank through mud and gravel. Their bodies were pushed into the deep places beneath the Earth. For a millenia the world stank of their ruined corpses and the whole of it was their sepulcher.

But memories pile on memories, and dust on dust.  Glaciers rose and sank. Seas parted. Forests bloomed and burned and from the ashes, bloomed anew. In the middle of one forest, a particular tree found purchase in loamy soil. Its canopy dwarfed all others, and its roots plumbed impossible depths.

It was the deepest taproot that pierced the cold carapace of the horrible thing. Woodflesh mingled with chitin. Conveyed upward, the vanguard of Things climbed root to trunk, trunk to branch, branch to leaf and for the first time in unknowable millenia shared the sky with the sun and the stars.

From inside to out the thing--all other names long since battered to sand on the tide of ages--choked the life from its benefactor. Leaves fell and branches withered.  The trees saw the greatest of their number weaken, slacken, and decay, and they girded themselves for a time when her mighty corpse could no longer contain the horror waiting to escape.