Complex sentence: He is eventually revealed to be the younger brother of Zeref Dragneel, who revives him from death as an demon known as "Etherious Natsu Dragneel" (E.N.D.), the master of the dark guild Tartaros.
Split into simpler sentences: He is eventually revealed to be the younger brother of Zeref Dragneel, who revives him from death as a demon known as an. Natsu's full name is "Etherious Natsu Dragneel" (E.N.D.) and he is the master of the dark guild Tartaros.