Right, i've been thinking this for a while, that out of that whole prison, there was only five survivors. I really want there to be another prisoner, a serial killer, inside the tombs, that is strong and crazy. The group can come across him, because since they've got more people, they search more of the parts of the prison and come across this scary man.
It doesn't even need to be a prisoner, maybe a civilian, or a guard or even the warden.
What are your thoughts?