Supposedly drops off of Murk-Veined Baneberry, from Voidwatch.
http://wilddragonchase.com/ffxi/voidwatch065.png
^Chart similar to dynamis chart, don't know how accurate it is.
Too bad it's completely horrible and outdone by just about every other GS, including the Gram off the auction house and of course destroyed by Caladbolg, Ragnarok and the OAT Magian GS.