Did any military leader ever make his own army fight itself and keep only the victors?
Did any military leader ever split his own army into two groups, force them to fight each other, and then keep only the victors or survivors as elite warriors or something of the sort?
I am not asking about pure punishment practices like Roman decimation. I mean a case where this was done as brutal selection, training, or hardening to make the remaining warriors tougher, more loyal, or more battle-ready.