Zeref Dragneel

The Black Wizard Zeref is the most evil wiazrd in the Fairy Tail universe. He is known to be still alive and sealed in a multiple lock prison. He is trying to be set free by using his loyal followers who are now the Dark Guilds.

"Hate is the reason for my existence"

- Zeref meeting Gerard, Chapter 81

Background
Zeref is known to be from ancient times and that in his present state, he can only be seen by people with a lot of hatred at the moment. At one point, he used a young Gerard to help him get rid of one of his prisons keys. In the long run he was successful in his plot.

Urtear seems to be his most loyal follower.

Living Magic
Zeref's magic is called living magic. It apparently is 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 continue his evil unknown quest. Most of his creations are giant demons.

Lullaby
A flute that can transform into a giant demon, whoever hears Lullaby's song (aside from the caster) will die. Eligor hope 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 Dragonil, Gray Fullbuster, and Erza Scarlet were able to defeat the demon flute. It currently is in the Council's hands.

Deliora
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. Leon Reitei tried to unfreeze him so he could kill the demon, however, after years of being imprisoned in Ur's ice, the demon had already been dead for years.