Zeref

ゼレフ

The legendary dark mage Zeref is a human wizard known to history as the strongest and most evil mage to ever walk the earth. He lived over 400 years ago, but even to the present day his legacy continues in the form of other dark wizards and dark guilds who worship Zeref's power. In fact, most dark guilds were founded and operate under the pretense of reviving Zeref, and helping him to conquer the world. Not only was he an incredibly strong mage, but he was also regarded as a genius, and he used his immense intellect to try to research and discover new magic, regardless of his status as the most powerful human alive. He is also responsible for the creation of tens of thousands of demons by means of his living magic, and these demons continue to roam the earth, leaving trails of destruction behind them. Aside from living magic, Zeref was also a skilled user of death magic.

Spoiler

Later on, it is discovered that Zeref found a way to achieve immortality and has been alive for over 400 years, however on top of that he has also picked up the undesirable ability of killing everything around him regardless of whether or not he wants to. He has taken residence on Tenrou island to avoid contact with other humans so that he can't kill them.
Tartaros, one of the 3 dark guilds of the balam alliance is comprised completely of demons from the books of Zeref. Although the leader of the guild is supposedly E.N.D (the strongest of all Zeref's demons), he has yet to be summoned. As a result, Mard Geer the leader of the underworld is the guild master. Mard Geer reveals that the reason Zeref created the demons of the books of Zeref (dubbed the Etherious) was to relieve Zeref of his immortal life and successfully kill him.
At an unspecified point in time, Zeref created the Alvarez empire, which is an empire comprised of all 730 magic guilds of Alakitasia that Zeref brought together by means of martial force. There he is known as Emperor Spriggan. He is regarded by his citizens as a great and kind ruler, but also a fearsome and powerful one. He created an elite group of the most powerful mages in the empire, and they bear the name the Spriggan 12.
400 years ago, Zeref's parents and little brother were killed by dragons. He then proceeded to study magic at Mildian Magic Academy where he studied the connections between life, death, and magic in the hopes of resurrecting his lost brother. He continued to study this and created the R-System and Eclipse Gate against the wishes of his superiors, who feared the wrath of Ankhseram (the god of life and death). 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 Zeref's 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 Zeref's 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 Zeref's 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, 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).