A fictional devil of vengeance and chaos in Paradise Lost. His name in Hebrew language means "fierce lion." Originally the guardian angel of Enoch, when the morning star Lucifer (or Satan) had started to rebel against God Arioch followed and became a fallen angel.
"Origin: Israel. A demon that presides over swords and chaos. He is also said to be the demon that holds sway over revenge. In Hebrew, his name means 'Fierce Lion' or 'One Like a Lion.' He was Enoch's guardian angel, but fell from grace when the angels revolted. He is the keeper of demonic swords, such as Storm Bringer. He appears as a being with black bat wings, with a flaming torch and a bloody axe."
—Shin Megami Tensei A-Mode DDS Dictionary
Along with Surt and Astaroth, Arioch is one of the three major sub-bosses of the last arc of the game and one of the most powerful demons under Lucifer's orders that has taken control over the inferior part of the Basilica.
He appears in the fourth floor basement, fighting anyone that follows the will of YHVH because of his feelings for vengeance over "[what He] did [to him]". It's unknown what he means, but he is probably referring to his grotesque appearance, suggesting that it was caused by some kind of punishment from God.
In Chaos route, after locating the throne room of Asura in the eighth basement, the hero will need to go back to Arioch's room, where he will give him the "Arioch's Fang" - a valuable item needed so Asura can forge the "Devil Ring" and make the fight against Michael and his followers become available.
"A fallen angel of Judeo-Christian lore representing vengeful spirits. His name means "Fearsome Lion." It's said that he only helps get revenge for those who hire him. When he still served God, he was entrusted with the protection of St. Enoch. He is originally mentioned in the books of Genesis and Daniel, but somehow he came to be known as a demon."