Location from Memory, Sorrow, and Thorn by Tad Williams
The Old Heart — a primeval forest covering central Osten Ard, hiding Sithi ruins, dream-walking witches, and the last settlement of a dying immortal race.
The Aldheorte — 'Old Heart' — is the forest that mortals walk through quickly and Sithi call home. It covers much of central Osten Ard and functions as a natural barrier between the kingdoms. Mortals avoid the deep forest with good reason: the trees are older than their civilizations, the ruins of Sithi cities lie half-buried in the undergrowth, and things move in the green gloom that don't answer to any mortal authority. Simon wanders the forest's edge after fleeing the Hayholt, and it nearly kills him. But the forest also shelters Geloë's lake-house, the abandoned Sithi city of Da'ai Chikiza, and — deepest of all — Jao e-Tinukai'i, the last living Sithi settlement. The forest doesn't choose sides in the war; it simply continues, ancient and indifferent, as it has for millennia.
An immense, ancient forest — trees so old and tall they block the sun completely in places, creating a twilight world of moss, fern, and silence. The canopy is a cathedral ceiling. The undergrowth is a labyrinth. Streams run cold and clear between root-walls taller than a man. In the deepest parts, where the Sithi still live, the forest becomes something else entirely — luminous, singing, responsive to presences that aren't quite physical.
Also known as: Aldheorte, The Old Heart, The Great Forest