Explanation: Decimation was an extremely rare penalty in the military of Ancient Rome. A unit which had dishonored itself in some way - usually disobedience to direct orders or cowardice - would be gathered, shamed, and then required to split into groups of ten. The ten men would draw straws - the one who drew the short straw would be beaten to death with clubs by the other nine.

This was both punishment - as no one wants to be the one to be beaten to death, and no one wants to beat one of their own comrades to death - and redemption, as by executing (ha) the order to kill the 10th man, the other 9 shamed troops are symbolically re-submitting to military discipline and 'cleansing' their unit of guilt by being the instrument of punishment themselves.

Whenever it happened, it was a big enough deal that contemporary historians would note it - not simply some humdrum everyday occurrence.