User blog comment:IamJakuhoRaikoben/Chapter 422: Orochi's Fin, Review./@comment-24744891-20150302140037/@comment-7418318-20150302165757

It isn't the Orochi Fin mark, but neither is it the Grimoire Heart... or any other known mark for that matter.

It could be something other than a guild mark too for all we know. Just a regular tattoo or a spell-tattoo.