Travel to Silithus and search for a Reliquary of Purity within the ruins of Southwind Village. If you are able to find it, return with it to Rabine Saturna in Nighthaven, Moonglade.
The first step is to find a container capable of holding an extremely corruptible substance without the handler suffering ill effects. There is only one sort device I am aware of that could possibly contain something as vile as the Felvine: a Reliquary of Purity.
Deep in the wasteland of Silithus is the remnants of the night elf village known as Southwind. I know one was there many, many years ago. If we are to succeed, I need you to scour those ruins in the hopes of reclaiming it.