Decimation was the most extreme punishment of the Roman army, where a tenth of a unit that had proven its cravenness was killed. The Romans believed that it had ancient roots in the early republic – the fifth and fourth centuries – and the theoretically unlimited powers of the Roman magistrate militae ("on campaign" ).[2] The procedure for decimation, as described by Polybius, involved a soldiers' assembly before the tribunes. Then, of the units adjudged cowardly, lots were taken such that a tenth of the men were condemned. Their comrades then killed them with clubs before the survivors were then further punished with barley rations and required to bivouac outside the fortified camp.[3][4]

The practice was rare: there are only eleven recorded instances. Five of them date to the late republic from its revival in 72 BC with a further four during the triumviral and civil wars prior to Actium.[5]

https://en.wikipedia.org/wiki/Decimation_(punishment)