Moonwillow Peak is a serene grove overlooking the Nightmarsh to its west in Shadowmoon Valley. During the Shadowmoon Valley storyline, Prophet Velen and the Alliance Commander come to this location to drink of the Clarity Elixir. Before the oracle stone beneath the great willow, they share a vision of the Iron Horde attacking and defeating the draenei of Karabor using the power of the Dark Star. By the end of the storyline, the oracle stone is no longer located beneath the willow.
Given the presence of tree roots covering the Altar of Shadow in Shadowmoon Burial Grounds, and the fact you enter the instance from the east and then turn left (south) as you descend, it is possible that the Altar of Shadow is directly below Moonwillow Peak.