A seclusion of life or death. Once entered, the result could only be one or the other.
Normally speaking, one would choose to go into a life-or-death seclusion when they hoped to break through an entire stage and increase their lifespan. Without Poji Pellets for their level, however, and without any scripts or manuals for the next stage, such a breakthrough was often at times impossible; out of a hundred of those that choose such a seclusion, almost none of them come out. Those who had successfully completed such a seclusion by breaking through only existed in stories and legends.
All things aside, the only good thing about such a seclusion was that no one would know whether the person inside was alive or dead, and the uncertainty would keep eager enemies at bay for quite some time. This was usually the last thing a powerful cultivator could do for the sake of their sect or clan.