We are being hasty here.If that guy was E.N.D,why was he introduced as the "king of hades" and not as "E.N.D-the king of Hades"...I think that guy is some kind of Bluenote,he is the link between Kyouka and the nine gates of hell with E.N.D,in other words,he is the second in command.
The one he called master is probably E.N.D,it is the book in his hands.Also, Lamy said that through the "king of hades" 's powers,tartarus was immortal as long as "Hell's Core" was active.She didn't say E.N.D...
Also,Kyouka said that E.N.D was sealed...I imagine the seal is the book itself.The King of hades said master,and it closed in on the book."Zeref's bookshelf"-Tartarus...I imagine each demon is a book.If that is the case,there is probably no way that the king of Hades is E.N.D,for he is carrying what appears to be E.N.D's book form.
I imagine part 3 will end with Natsu and Lucy vs King of Hades...and E.N.D will probably awaken on part 4 or on a probable part 5.Well,the fight against E.N.D will probably get it's own part,since the fight against Hades was 5-6 chapters long...almost the size of one of the parts of the Tartarus arc:7-8 chapters.
If King of Hades is really E.N.D,that book isn't,and the master he was referring to was Zeref,then just ignore my ramblings.
According to mangastream,this guy is called Marde Guille "the king of demons".
Could the "King of Hades" mentioned in 379 be him,or is it E.N.D...I still believe it is E.N.D...