Find the Felvine in Dire Maul and acquire a shard from it. Chances are you'll only be able to procure one with the demise of Alzzin the Wildshaper. Use the Reliquary of Purity to securely seal the shard inside, and return it to Rabine Saturna in Nighthaven, Moonglade.
Your knowledge of Eldre'Thalas - Dire Maul - in Feralas will now come into play, <name>. In the eastern part of the ruined city is where Alzzin the Wildshaper has exerted his corrupt dominion. Go there with all due haste and brave whatever challenges await you.
The Fruit of Fertility, now corrupted as the Felvine, should be close to Alzzin. When he falls, capture whatever essence of the Felvine you can. Seal it in the Reliquary, and bring it to me.
With the Sealed Reliquary of Purity in our possession, it will be possible not only to remove a corrupting threat from the world but also to study it under our judicious watch. Perhaps we can even devise new means to combat the taint of corruption that ever threatens Azeroth.