Etrigan

1st app- The Demon #1

When Camelot seemed to be lost, Merlin decided to summon a demon from hell to fight the forces of Morgana Le Fay. The demon Etrigan fought bravely against the woman's armies, but Camelot was already doomed. Merlin then put a spell on him to turn him into a human and he walked out of Camelot as one of the defeated knights
Etrigan lived as a human for centuries and took the name Jason Blood, probably believing himself a human. He finally moved to Gotham City. Finally, a spell allowed Blood to turn again into a demon. Thus, Morgana discovered him and sought revenge. Realizing that she was no match for the demon, she enslaved a weakened wizard and made Blood believe he was his old master. For years, the old man played his rol until he made Blood drink a special potion. It was supposed to free him of his memories, for he loathed his demonic self, but instead it induced false ones to make him serve LeFay. The only flaw on her plan was that her spell make him speak in rhyme. Unaware of LeFay's plot, Blood eventually tried to conciliate his two personalities, but his human self became colder and more unhuman and his demonic self became brighter and more ruthless. However, it is possible that the demon is now more human, for he helps people now. He even helped Constantine, Zatanna and the Swamp Thing to fight the ultimate deadly and powerful foe who caused the death of Zatara and Sargon.
Tired of Etrigan, Blood stole his heart and bound him to his service. During the time, Etrigan stopped talking in rhymes but Tommy Monaghan, a hitman who needed his protection, helped him get it back. Then, Blood was fooled into separating himself from Etrigan. Blood became a lifeless shell and Etrigan was de-humanized. By the time, Morgana LeFay had possesed Helena Sandsmark and her daughter Wonder Girl, Artemis and Gateway City cop Michael Schorr had to go to London to free her. They failed and were captured by the witch until a Merlin from a different timeline helped them. After their defeat, Le Fay, her corrupted Merlin and Etrigan were sent to hell. However, the heroes went there to free Etrigan.
Basically, Blood is a student of the occult and a wizard. However, his true power is in the shape of the demon. Then, he possesses a superhuman strength and he can cast spells ranging from destruction to teleportation. His alter ego, Jason Blood is an immortal and also possesses the ability to cast spells. They both can sense the future of the people they meet.

Credit- CCDB