The Tainted Waveling is the prize you're after, though I suspect some of you may have one already. You are, of course, seasoned veterans of Pet Battles. So let's hear your stories of triumph either from your adventures in Serpentshrine Cavern, or the tiny battlefield with your poison-spewing companion!
I should note that you don't actually have to take down Lady Vashj, but she does deserve some righteous smiting for her tainted actions. (See what I did there?)