Omen is an instrument of the Dark Ones. The ones who want to see humanity destroyed so they can have free reign of humanity and the world again. Since they are not able to intervene directly, they influenced humans that worshiped them. They found a way to give some of their power to a human without it being considered an intervention. Thus Omen was born.
Omen was put with the task of corrupting humanity. She has powers beyond those of normal mortals, and uses them to tempt people to evil. Because Harbinger was created soon after to counter the evil influence, Omen tries to stay in the shadows so as not to become a direct target of Harbinger.
As they keep hidden, it is hard to locate one. However, Harbinger’s father found one. She was working with the Nazi’s during World War II. He had found that she discovered he had a son. They both knew that his child would be the next Harbinger, and she planned to kidnap him and corrupt him before he could ever get his powers. That is what led to Harbinger being raised by monks. He was being hidden until it was his time to protect others.
The last known Omen was an illusionist. As such, it was hard to track her down, and when confronted, she could always get away with a illusion distraction.
It is not known if she is still the current Omen or if the powers have passed onto a new one. However she has not made her presence known, but that is her way, so she could be behind any plot you are confronting.