Zeref Dragneel

The Black Wizard Zeref is the most evil mage 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.

History
Zeref is known to be from ancient times and presumably wields nearly unimaginable power. Ultear seems to be his most loyal follower and impersonated him to control a young Jellal Fernandes. It can be assummed Zeref is very powerful considering that the strongest wizards in the world, the Wizard-Saints, shivered hearing his name. Somehow Zeref may have achieved immortality, as he is stated to still be alive, just trapped in his prison.

Magic and Abilities
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 and continue his unknown evil quest. Most of his creations are giant demons.

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 Dragneel, 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.)
 * Lullaby (ララバイ Rarabai)

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.
 * Deliora (デリオラ Deriora)