Tartaros. A guild-like or a group of demons that came from the Book of Zeref. They are known to be following the Underground King, Mard Geer as Mard Geer follows Zeref Dragneel's, the Tartaros almost succeed of reviving their original master, E.N.D They'd follow from the Book of Zeref so that they can come back from Zeref Dragneel to E.N.D!