He is the older half-brother Valerie LaMarche, the Expression practitioner of New Orleans who tricked Damon Salvatore into committing an Expression sacrifice. Unlike his half-sister, who died of old age, he was far more immerse into the dark forces of Expression and found a way to extend his natural life span much further than with ordinary magic. That way turn him down the dark path of demonology and voodoo.
While true demons don't exist on Earth ever since they were banished into hell dimensions by the first witches, they can be brought forth to Earth by Expression practitioners. Durza perfected a ritual that allowed him to reach out through The Great Wall to get possessed and unite himself with a powerful ancient Demon.
However, Durza did more than just allow himself to be possessed, his ritual was powerful enough to enslave the Demon within him. That granted him immortality as he won't age as long as the demon is inside him but if it were to be exorcised at his current advanced age, he would lose the only thing keeping him alive and die soon after.
He is very powerful but because of his demon based magic, blasts of concentrated light magic and angelic powers are extremely harmful to both the demon and his master.
He is quite sadistic and doesn't care who he hurts as long as he gets what he wants. His immerse exposure to Expression magic left him almost entirely void of compassion and while his is usually in full control of his inner demon, he has lost control several times over the decades and when he does the Demon uses his host to go on a rampage of carnage and destruction that would scare Mikael Mikaelson himself.