I recently presented my party with a situation that could have led to a total party kill (TPK). The situation was this: The party was being followed by two assassins who also had rudimentary spell use. One of their spells was Charm Person. They tailed the party as they returned to a large city. The party keeps a watch at night as they travel, to protect against any attackers that might come upon them while they camp, three four-hour shifts, usually two characters per shift.
Now I, as DM, would have no qualms about the assassins trying to charm the two characters on watch, as success would likely leave the rest of the sleeping party open to fatal attack. It is my job to have the killers, who are smart, do smart things, like wait for the best chance to use their spells, then slay the sleepers.
It is happy for the party that saves were made and the threat did not materialize. It was at this time that one of my players tried to re-assure the other players by saying that the DM would never let the sleepers die without a chance to save themselves. That I would have an assassin step on a dry branch and give the PCs a chance to hear it. Or that one of their animals would make noise and cause a ruckus.
I denied this. I made it clear that the threat of death was real. In this case, their chance to escape the TPK was in making the saves that they made. The Assassins were good enough to not step on a dry twig and capable enough to deal with common animals. It’s not my job to save the party in this or any other instance.
Moreover, it IS my job to instill dread and fear. The threats they face must be real.