Graham White

 is considered to be an extremely powerful witch, one that even Klaus fears. He is introduced towards the end of the series, after Klaus recommends him to Bonnie to help deal with her curse. Aside from his magical power, he is also quite wise, and has deep knowledge about the origin of witches and witchcraft.

Early Life
Klaus and Graham's paths crossed in New York, Manhattan, during Graham's youth. Realizing his extraordinary power, Klaus tried to trap him and then save his life, making him his witch through gratitude and life debt. But Graham realized his play and fought back to Klaus, which led Klaus to hold a certain level of fear and respect towards Graham.

Graham's powers and deep knowledge about witchcraft led him to be able to contact his ancestors, even as far back as to first witches. Everything he learned added more to his strength. He is also shown to have no problem with inviting vampires into his property, since - according to him - with the proper spell and blood from the invited vampire, it is possible to rescind the invitation.

Season 6
Graham was first mentioned by Klaus, when Bonnie realized she's cursed with the Elemental power of the River Witch, Irene. Gang went to visit him hoping he might find a way to lift the curse.

Trivia & Notes

 * Graham knows and reveals the origin of witches and witchcraft.
 * Graham also knows and respects Elena's mother Isobel Flemming, as he was one of the people she's seen while investigating witches.
 * Graham implies he is not slowing his aging, feeling that it is like cheating god.