Slay the insane druids: Taneel Darkwood, Uthil Mooncall, and Mavoris Cloudsbreak.
The druids are dead, killed by their leaders who were driven insane by the Horde tainting the Forest Heart! I can feel the corruption even from here and I dare go no further.
While I am too well attuned to Ashenvale and the Forest Heart, you... you could descend into the barrow den and do what must be done. I've laid down breadcrumbs for you in there.
Though it tears me apart, I must ask that you put to rest the druids' once great leaders: Taneel Darkwood, Uthil Mooncall, and Mavoris Cloudsbreak.