It's the jungle. Trapped in the walls of Root Sunder - it isn't a Daedra or ghost. It's the jungle itself. The Elves here found a way to bind the spirit of the jungle to the city's Welkynd Stones. They must have thought themselves brilliant at the time. Bind the jungle to the city walls, then control the walls with magic. No more randomness. Pure order. But the jungle went mad, trapped within the artificial walls. It took control of the city. Killed the inhabitants. Dragged the city beneath the ground. Then it slept. And we woke it up.