Decimation

Hjalstone massacre » Decimation
An old Anuirean punishment for a mutinous legion was to order every tenth man (drawn by lots) killed. This was done only under extremis - more normally ringleaders of any trouble would be identified and slain; however if the ring-leaders could not be identified, or a message needed to be sent and if the crimes of the legion were extraordinarily severe, the command could be issued that the legion was decimated.
A legion selected for punishment by decimation was divided into groups of ten; each group cast lots, and the soldier on whom the lot fell was executed by his nine comrades, often by stoning or clubbing. The remaining soldiers were given rations of barley instead of wheat and forced to sleep outside of the encampment for three nights - a particularly harsh punishment in the Rjurik lands.
Because the punishment fell by lot, all soldiers in the selected legion were eligible for execution, regardless of rank or distinction. As a result, the threat of decimation inspired fear and resolve into the Anuirean legions. However, because a decimation significantly reduced the troop strength of an army and crushed morale, it is believed that the punishment was rarely used.
As a note, barley does not rise when used to make bread in the same way that wheat does and is therefore symbolic of the legion's disgrace, similarly sleeping outside serves primarily to isolate the legion and show it is 'outside the pale' prior to being accepted back after three nights when sufficiently penitent. That they are also more vulnerable to attack and will be far less comfortable is really a side effect of the punishment than the punishment itself - the main punishment for the 'lucky nines' is being forced to slay their own.
Elements of this page were originally based on text from Wikipedia.