Its trees are watered on blood and yearn to sink their roots into meat - without the appropriate safe passage, any non Sylvaneth who enter the wood will be attacked. The wood itself hears the song of Drycha Hamadreth rather than Alarielle. 
In the Age of Chaos, Drycha slew Duke Goral of Festerfane when he sought to destroy the Weald and entombed his body and his Axe Lifebiter at its heart, leaving her followers to guard it. The axe tainted its heart and in the Age of Sigmar Bramble-Maiden bid Sir Roggen extract it and bring it to her.
|These spirits do as they will. It is not Alarielle’s song that they hear, but a rather more primal melody, and darker by far, sung by one whose will is as tangled as the roots of this forest. You know of whom I speak. That they have not plucked out your eyes and made a toy of your beast’s pelt to taunt you with as you wander blind and anguished is proof enough that you have safe passage.
~ A spirit to Sir Roggen.