Dreaming Wood

Known as Athel Caiellin in Fan-Eladrin, the Dreaming Wood is not so much an area of Athel Loren as it is a wholly separate region that sits alongside the entire forest. It is a realm of grim prophecy and stark terror, of endless magic and daemonic malice. The Wood Elves enter the Dreaming Wood only at times of greatest need, for they know well that there is no surer way to draw thirsting Slaanesh's attention. The Dark Prince's servants ever prowl Athel Caiellin, searching for errant Elves, and battling with others o f their kind.

Athel Caiellin can only be accessed in places where magic lies heavily upon the land. Many of these sites fluctuate with the passage of the seasons, and only serve as portals at high summer, or in the darkest depths of winter. Most of these locations are marked by archways of vine-tangled white stone, placed as warning to the unwary that the Dreaming Wood lies beyond. Others are given away only by dark whispers on the breeze and a slight shimmering of the air, beyond which the glades take on a strangely sinister or unwelcoming aspect.

Source

 * Warhammer Armies: Wood Elves (8th Edition) -- pg. 12