This reported massacre, or string of mass-killing set amongst over a week of battle (August 18-26, 2012), allegedly occurred in Al-Harak, near Daraa. A death toll of over 500 was reported, including rebel fighters but, according to them, only a few. The UNHRC's investigation reported "the FSA (Free Syrian Army) source claimed that Government forces killed 565 people, of whom 30 were FSA fighters," and the rest innocent civilians.  These included women and children, many executed with guns or knives and burned, they said. But some were also killed in shelling and cross-fire. The approximate alleged proportions are unclear.
The period of the alleged massacre coincided with the successful government re-conquest of a Al-Harak, which rebels had taken over in May in order to launch attacks on the nearby military base. Many of the bodies from the massacres are buried in a mass grave inside the base, rebels say. Thus, there was not as much proof of their claims as there should be. There seems relatively little in the line of visual evidence shared publicly, altohugh one fighter told the UN's investigators he personally saw at least 400 bodies of innocent civilians. The UNHRC's sources included rebel fighters, but not a single government soldier. From this sampling, they found "reasonable grounds to believe that Government forces and Shabbiha, committed the war crime of murder, killing persons hors de combat and civilians" in Al-Harak, and no such evidence for rebel fighters being complicit in any of it. 
This page, currently a stub, will proceed on the assumption that the UNHRC's findings are worth close and critical examination. See the discussion page until there's enough to fill the page in.
The huge reported death toll amidst a rebel military defeat is a trait shared with the "massacre" at Daraya, a major Damascus suburb, in those same days. This too had a reported death toll of at least 4-600, featured some women and children but mostly fighting-age men, again coinciding with another rebel defeat. In Daraya, it seems, the rebels were able to inflate the death toll with their own mini-massacres of locals taken hostage during their weeks in control of the city. The same possibility cannot be discounted as playing into the horrible death toll in al-Harak.
Both of these very similar alleged massacres, Daraya and Harak, were singled out in UN Human Rights Council's February, 2013 report, on their short list of five mass killings by pro-government forces investigated. (this is since their last report of June, 2012, and some four others were still under investigation).