Upon taking command of his Legion, Perturabo reviewed the war record of his new forces. After heavily analyzing their record, effectiveness, doctrines, and practices, Perturabo found them wanting. His punishment was decimation. By lottery, one in every ten Legionaries was chosen to be beaten to death by his comrades. Such would be Perturabo’s reign: brutal and unforgiving.[20a]

from: https://wh40k.lexicanum.com/wiki/Iron_Warriors