It happened during the Donatist Schism of north west Mesopotamia in roughly 3rd century AD. The emperor at the time, Darius Scoliosis, didn't like the peasant people who the Mesopotamians had brought from Asia Minor to work on the farms of aristocracy who decided they mattered too much to be doing manual labour. When I say he didn't like I mean he literally detested their existence. For example in his memoirs he is quoted saying 'the arrival of the asians is disgusting. They smell like half cooked cheese and they look like they came out of a goat's ass.' He just hated them.

With this hatred as the pretence he ordered a small council of the most noble Mesopotamians called the 'Intolxocia.' In this they decided what they should do with this 'disgusting' new people. In Scoliosis's memoirs he quoted a nobleman who was at the meeting saying 'We should round up those swine and turn them into ashes.' This general sentiment of hatred was held by everyone at the meeting and so they did just that. They literally went around to all the farms and found the slaves and burned them then and there on the spot.

Source: Darius Scoliosis's memoirs, am Historian majoring in genocide.