When the council of angels threw Lucifer out of heaven after rebelling against him, They assigned a group of angels the task of watching over humanity to guide it along the righteous path. This group, called the Watchers, broke their oath and fornicated with man, creating a race of giants called the Nephilim. The leader of the Watchers, named Azazel, taught man the weapons of war to give them a fighting chance against their monstrous offspring. However, the council disapproved of this action, and cast the Watchers out of heaven, never to return. Rejected by heaven and by hell, this group set out to create their own destiny in the cosmos.
As heaven is inhabited by angels and hell inhabited by demons, the Watchers took up residency in purgatory as their new home. From there, they would take up arms as celestial professional mercenaries in the holy war, accepting jobs from various employers of different factions. They would no longer serve any master but themselves, and work for the highest bidder. As they have no loyalty to any one side, they can end up working for both sides of the conflict at various times, killing demons or their brethren depending on who was paying them at the time.. They would even export their services to other entities of the ongoing celestial war, such as Moloch or Baal. This state of affairs has gone on for millennia.
Although they are an effective fighting force, they are ultimately a small army playing a dangerous game with larger, more powerful forces. It would be simple thing for someone to see them as a threat, deciding to exterminate them to prevent them from helping their enemies, or for various factions to team up and destroy them. How can these mercenaries maintain the status quo while continuing to operate as they do?