You unlock this achievment by completing the bad ending of the game.
To do so you should progress the game until you have acquired the mortal blade, beaten the
. After this you need to return to Ashina castle. Upon reaching the top of the castle you will be confronted by a choice. To get locked on this ending path you must choose to uphold the Iron Code and forsake your master. Following the choice you will engage two bosses back to back. Upon defeating them you have unlocked the Shura ending and the achievment.