Some creatures are utterly fearless, and will never give up a battle, no matter how hopeless the situation. This is occasionally due to bravery, but more commonly because the troops in question are mindless, insane or magically controlled.
Models with this special rule have the Immunity (Psychology) special rule and pass Break tests automatically. However, they may never choose Flee! as a charge reaction. Pride, or a sluggish acceptance of the situation, prevents them from doing so.
Characters that are Unbreakable may only join units that are also Unbreakable, and characters that are not Unbreakable are not permitted to join units that are Unbreakable (even if a character is Unstable – explained below – or was to become temporarily Unbreakable for some reason).