Batman has long been a man of science and reality, but throughout his life, he has been taught that sorcery is just as prevalent in the world. One just encounter led him to a man named Jason Blood, a being cursed to near immortality and bonded with a demon called Etrigan.
In many continuities, Blood was one of the Knights Of The Round Table, and was conned into helping bring down Camelot, he was cursed by Merlyn and made to walk the Earth in order to repay for his sins.
Eventually, he found Batman, and the two made an odd alliance. In the DC Comics cartoons, Batman and Blood are actually good friends, and Batman knows to trust the demon Etrigan, despite his dark appearance.