An ashen grey willow with pale leaved tendils obscuring it's gnarled trunk. Sitting in the mire of great swamps, rare even in it's natural environment. The Banshee Willow's bark is known to have magical properties. Travelers beware, these trees seem to attract ghosts and other unsavory undead.
It seemed like a great place to camp. The clearing was good sized and sheltered from the wind. The brook just a few feet away. There is a natural hallow to keep the horses.
Then the night came.
It was like it became a different place. The temperature dropped. The wind, which does not seem to disturb cloth, almost cuts through you like an arctic wind. No one can sleep, as the soft ground has turned hard. The horses are uneasy. The Bats are flying over and stopping in the trees.
And then there is the eyes. There are glowing eyes just inside the tree line watching your group. The mages and clerics can detect nothing, but there is still something there.
(yet there is nothing at all... The Darkness will do nothing unless the players do something to it. And even then it will all seem to be a conincidence.)
Of course, in the morning, it all becomes sweet and light.