Merlin is the son of The Devil and was meant to be the Antichrist, but he declined the offer and went his own way. He was very fond of Arthur and helped him build Camelot, ushering in a Golden Age the likes of which was never seen before and never seen again. They were as close as father and son. Arthur died trying to defeat Mordred while Merlin was chasing down Morgana. By the timehe had killed her, he was too late and Arthur was dead.
He was drinking in the oldest bar when John Taylor and Shotgun Suzie and Tommy Oblivion went to see him. They instead convinced the druid priestess Nimue to drug Merlin and then they cut out his heart.
Arthur's halfbrother tried to kill Merlin and Nimue gave her life energy to save him; she died. John Taylor and Suzie attempted to use the heart but Tommy tried to stop them, fantasizing about a Camelot revived. He failed and was sent back to the present by John. John and Suzie took Nimue's body further back in time where they dumped it and went on their way.
Merlin believes that Nimue stole his heart.
Merlin helped in war against the Lilith, later when Lilith invaded Strangefellows´ she came into bar clutching Merlins heart her hand. Lilith challenged him to worship her and Merlin laughed to her face, so Lilith crushed his heart in her hand, and chewed on piece of it, while watching him dieing his final death.
Merlin is a sorcerer and, as the son of the Devil, has very potent magic. even after his heart is cut out and he dies, Merlin exists, powered by his awful magics and the spell he placed on his descendents. Buried under the oldest bar in the world, his will made manifest can take over the body of his descendents and bring him forth into the world for whatever purpose he desires and however long he chooses.
He was taught by Herne the Hunter in his youth.
He also created powerful artifact called Merlins Glass for droods, most recently used by Eddie Drood.