During the war the Department of Mysteries' secretive off-shoot, Q-65, developed their own response to the threat that was Voldemort and his war. The Sleepers unit was established in the fall of 1999 as a covert ops infiltration and extermination squad. It was formed of the most elite soldiers they could get their hands on who were muggleborn and muggle both. Muggles were approached who were ex-military, high-ranking combat specialists with a proposal. They could join up, fight a war no one knew was going on, see things they would never have a chance to see in their lives, keep all of England safe even though no one would ever know it and then lose their memories they'd ever taken part in such a thing. They would be paid well and in the case of their deaths their payment would go to any person of their choosing. A fair few thought the men who approached them were completely crazy. There were a few, however, who could think of nothing better. These men tended to come from reconnaissance, counter intelligence, and the elite of British military operations. All were combat-trained and weathered. These volunteering muggles were introduced to the wizarding world that existed cloaked around them. The seven ex-military muggles joined another eight muggleborn witches and wizards and formed the Sleepers unit.
The Sleepers used muggle weaponry, high powered, semi-automatic and automatic guns as well as semi-automatic pistols, hand grenades and explosives. All of these weapons were altered magically. Ammo had invisibility and camouflaging charms cast upon it and silencing spells were put onto every single weapon they used, particularly the guns. The Sleepers had a single task: Infiltrate Death Eater strongholds and kill as many of them as you possibly can. No regard was given to who may have been under Imperius and who was not. People had lied before and considering some of the things the Death Eaters had done during the war it was perhaps a gentler fate for those who genuinely were to not live with the memories of their actions. The Sleepers found Death Eater camps and snuck in under the cloak of night. They murdered Voldemort's loyal in their sleep without causing a sound. During battles they would be positioned far away, under cover and pick the Death Eater's on brooms out of the sky or pick them off as they ran around the ground with long-distance sniper rifles.
The Sleepers were highly trained and only ever suffered two causalities, both were muggleborn wizards. The rest made it through the war and all of the muggles involved were thanked. They were awarded military honours from their own government for deeds they never even asked about. Their memories were taken, oblivated, to encompass their entire involvement with the magical community. The last memory they had was agreeing to the terms and knowing their memory would be taken. None have ever gone looking for answers, they simply take their honours and their payment as a sign they helped someone in need and succeeded as they can look around and see their own country safe and in one piece, though perhaps a bit more ragged than it had been from their last memories of it. The muggleborn fighters in the Sleepers unit either retired and left the country for parts unknown, or were absorbed into various Q-65 agencies, including PARS and the Spec Ops.
No one knows the Sleepers ever existed or what their contribution was to the war. The Minister awarded honours in private. Not a word has been spoken of them since.