Zeref

At the time, he was a very caring and smart boy, but then Ankhseram cursed Zeref with the curse of contradiction (the more he values human life, the more the people around him will die), as well as immortality. Since then he has searched the earth and created countless demons, even the Etherious so that he may somehow die, however he has been unsuccessful.

The time Zeref has spend under the curse of contradiction has slowly begun to take its toll on Zerefs mind, and he has begun to develop symptoms of a split personality, one personality being his original kind self, but the other being a mage who believes humans no longer deserve to live (as they have repeated the same mistakes of useless fighting and wars over and over throughout the many years he’s been alive).Over his many years, Zeref had encountered Fairy Tails first master Mavis on multiple occasions. They took a liking to each other, with Mavis denying Zerefs claim that he was a monster exactly like the legends of him say.

Mavis was eventually placed under the very same curse as him, and she grew into an unstable mind set. Zeref attempted to comfort her when she told him of this, and they decided to live together to search for a way to end the curse. They had fallen in love, but as they shared their first kiss, the curse of contradiction took effect, with Zerefs curse becoming the dominant of the 2 and consequently, Mavis is killed. Zeref was emotionally scarred by the death of the woman he loved, and he embarked on his seemingly endless journey to find death once again.Zeref eventually found a way to revive his younger brother, Natsu, and in doing so, also created a demon with the potential to kill him like he wanted.

The new demon was named E.N.D, Etherious Natsu Dragneel (Natsu being Zeref's younger brother, and Zeref's surname being Dragneel).