Cornelius Sigan

Cornelius Sigan was the greatest sorcerer Camelot has ever known.

Early Life
It is said that when he was alive he could turn the tides, change day into night and even helped build Camelot itself. His life's mission was to defeat death and he swore to one day return from the grave. Sigan became too powerful and the King at the time ordered him to be executed. Before his death, Sigan cursed Camelot, saying that he would one day return and raze the city to the ground. It was said that he didn't want his wealth and power to die with him and worked on a way to cheat death.

Return
This was revealled to be true hundreds of years later. After Camelot workers uncovered his tomb, a thief called Cedric worked his way up to a position in the Royal Household and used this to enter the tomb. He intended to steal Sigan's riches and jewels but found a large blue heartshaped jewel. The jewel contained Sigan's soul and he possessed Cedric, Sigan subsequently launching an attack on Camelot that forced Merlin to ask Kilgharrah for help.

After witnessing Merlin's magic, Sigan suggested that he and Merlin form an alliance, but when Merlin rejected the offer Sigan attempted to possess him. Using a spell that Kilgharrah had recently taught him, Merlin was able to trap Sigan's soul back in the jewel while it was in the air. Uther sealed up the vault where the tomb and soul of Cornelius Sigan lay.

It is possible that he could return sometime in the future, but it's highly unlikely.

Abilities
Cornelius Sigan was described as the most powerful sorcerer ever to live, while he was alive, he was said, to be able control the tides, and even change night to day. His powers were so great that he was able to imortalise his own soul, and keep it alive after his bodies death, by storing in a jewele.

Upon gaining a new body, his powers remained intact, as he was able to posses another, and could animate every gargoyle in Camalot and control it.

He was only defeated by an ancient and powerful spell that the Great Dragon told Merlin, that trapped his soul back in the jewele.

Behind the scenes
Cornelius shares similarities with the character of Lord Voldemort from the Harry Potter series by JK Rowling.