Zeref Dragneel

Zeref was the strongest and most evil wizard in history, who possesses a very dangerous magic. He can kill any living thing around him. This magic causes him strife so he lives all alone in the woods of Tenrou Island to protect people. Nothing else in known about him except he seems to know Natsu Dragneel personally and would like to see him again so he can have him kill him. His reason for wanting to meet Natsu is because he believes that Natsu is the only who can stop him. He is strongly hinted to be a Dragon Slayer.

In the past he was said to be cruel and evil. How he became who he is right now has yet to be revealed, but he claims that he doesn't want to kill anyone anymore. This might be a result of his evil side being "locked" somehow because Grimoire Heart is/was trying to find the keys to unlock/free Zeref.

History
Zeref is known to be from ancient times and presumably wields nearly unimaginable power. Ultear mentions that he, alighted an ancient land, mastered black magic, gave birth to tens of thousands of demons, and threw the world into chaos.

It can be assumed Zeref is very powerful considering that even the strongest wizards in the world, the Wizard Saints, shivered hearing his name. Ultear used Zeref's name to manipulate a young Jellal Fernandes. Somehow Zeref may have achieved immortality, as he is stated to still be alive, just trapped in his prison. He is later shown to be The Killing Wizard in the S-class promotion arc.

At some point, Zeref was defeated and sealed away for centuries. However, one day he apparently managed to set himself free despite the fact that Grimoire Heart is still collecting keys to his prison. Apparently, the slumber made him a very kind person as he cares for others' safety now and wishes to die. At some point in the past, he met Natsu and realize he could kill him someday. It can be presumed that he later wandered by himself, eventually finding Tenrou Island, and wanting seclusion, decided to have it as his foster home, completely unaware that the island falls under the jurisdiction of the Fairy Tail Guild (and no one else were allowed to step foot in it), and locking himself away from the rest of the world

Synopsis
Throughout the entire series, Zeref and his creations have played an influential part as antagonists of missions. Lullaby, Deliora, and the Tower of Heaven have all been somehow effected by his invisible hand. However, it seems that this is just because they were just acting on their own accord as he has been by himself the entire time. Still all the evil he did in the past has greatly affected the series.

S-Class Trial arc
After killing some wolves, Zeref apologizes and wallows in misery. Later, Charle receives a vision that he is somehow going to be involved with the Trial.

During the trial, he runs into Elfman and Evergreen and warns them to stay away from him as his magic starts to involuntarily activate. Natsu saves the duo as he is happy to see him but Natsu has no idea who he is. When he tells the Fairy Tail wizards that he has been expecting Natsu's arrival, Natsu misinterprets this as a form of challenge and rushed towards him, landing a fierce blow to Zeref's face. Although Natsu punched him, Zeref in unaffected and just stares at him. Zeref seems disappointed since Natsu still isn't strong enough to kill him yet. Just then his magic activates again but no one is killed, but Natsu's scarf (which protected him and others) turns black for giving itself up and he escapes somewhere. However, after using his powers, Grimoire Heart seems to have found him. He then sobs when he learns Natsu is not yet powerful enough to kill him.

Magic & Abilities
Known as "The Black Wizard". As a Ultear he alighted on ancient land, mastered Black Magic, gave birth to tens of thousands of demon and threw the world into chaos. Zeref is unanimously considered to be the strongest and also the most evil of all known dark wizards, if not wizard in general, to have ever lived. He was able to master all forms of dark magic at a very early age, and brought countless demons to life. His magical strength is so enormous, that in fact, that he cannot consciously control his own magic, though whether this was always the case is unknown. Even the mighty Ten Wizard Saints feared his powers. Given his sharp incisors it is very possible that he is also a Dragon Slayer, though this is speculation.

Death Praying: Zeref has the ability to cause any living thing around him to die when he uses magic. The name of said magic is unknown. This magic seems to burst at random times but he can tell when it is going to happen, so this gives people a chance to escape. Certain things can protect people from the Wizard's blast such as Natsu's scarf saving himself, Evergreen, and Elfman. It is implied that the magic activates whenever he is agitated, scared, or surprised.
 * Unnamed Death Spell: Zeref involuntarily releases a wave of Death Magic and anything hit by it dies. Like the rest of his Death Magic, he has no control over it.


 * Living Magic: Second part of Zeref's black magic, called Living Magic, is apparently a very ancient, long-forgotten form of magic that brings things to life under the master's control. Some creations of Zeref have minds of their own and continue his unknown evil quest. Most of his creations are giant demons.
 * Deliora (デリオラ Deriora): Deliora was a monstrous demon that attacked the icy villages in the North. According to Erza, Zeref created it. Deliora was imprisoned in a block of ice by Ur. Lyon Bastia tried to unfreeze him so he could kill the demon, however, after years of being imprisoned in Ur's ice, the demon's life force was drained so much that it died before Lyon could attack it.
 * Lullaby (ララバイ Rarabai): A flute that can transform into a giant demon. Whoever hears Lullaby's song (aside from the caster) will die. Erigor hoped to use this to kill the Guild Masters at their meeting. As Kageyama failed to carry out the mission due to being changed, Lullaby transformed into its demon form to feast on souls. However, Natsu, Gray Fullbuster, and Erza Scarlet were able to defeat the demon flute. It currently is in the Council's hands.(Its magic seal is what we assume Zeref's is, however this is likely not the case as even though it was controlled by Zeref, the Lullaby still had its own thoughts and will so it probably also has its own seal.)

Other
Zeref seems to be physically powerful as well. When Natsu, who is known for being strong, punched him in the face, Zeref just took the hit and somersaulted back on his feet in mid air. The punched seemed to have no effect on him and he had no bruise or marking on his face. This indicates that Zeref is not as frail as he looks.

Trivia

 * Charle has a vision of him present at the S-Class Wizard Promotion Trial.
 * It is possible Zeref is a Dragon Slayer who was taught magic by the Black Dragon itself.