Operational Assault Unit-16670 was a simple Omnic who was programmed as a guard for ceremonial celebrations and ancient temples. When the Omnic Crisis began, he was reprogrammed to turn against the humans and kill them. He wielded a glaive that he used to behead his targets, which many people called him the Head Hunter. Omnic-16670 was unstoppable, until when Overwatch was created, he was remodelled and given armoured plating to replace his normal plating, making him even more powerful.
During a year 16670 led an army of Omnics that attacked an unnamed city, until a strike team called Strike Team Mythos, led by Brigadier Henderson, aka Mastodon, who defeated the entire Omnic Army. Leaving 16670 alone, Mastodon used his power claws to slice 16670's head off, forever ending him. Or so he thought...
Years passed and the dormant headless corpse of 16670 laid buried in the remains of what he created. However, after many years, he was reawaken, and without a head, he was nothing but a headless robot. But his vision did function, through a special fusion core called a Soul Core, which turned him into a ghost. He returned, gaining his sanity before the war, but wanting revenge against Mastodon for taking his head. Taking a cloak and hood which was held up by a magnetic force, he donned the name Dullahan, the headless one.