In general, the most disruptive event you could have in a Dungeons & Dragons game is the Total Party Kill (TPK). That event where every member of the party dies.

One player character dying is a tragedy, but the campaign continues. However, if everyone dies, then it stops all the ongoing stories of the characters and could derail the campaign completely.

The possibility of a TPK is why mass damage spells such as fireball need to be carefully employed against parties. Use a fireball against a level 1 or 2 party, and that might be it for the campaign!

A single character’s death is a signal to the rest of the group that things are serious. And, as you play in a world of fantasy, the character can come back. However, bringing everyone back tends to be tricky.

Now, obviously there are times when you can make a TPK work for your group. There are many methods. Perhaps a new group picks up the quest where the original group failed and is inspired by the danger posed (as shown by the fate of the previous characters). Or you can run an “after death” campaign set in the afterlife where the PCs attempt to return to the world of the living. Or the entire quest is a failure, and the next campaign is in the world made darker by the failure. The Dark Lord is on the throne!

However, if you are designing an adventure for others to run, random TPKs are not that great, and you need to be careful in how dangerous your designs are.

I believe TPKs are a tremendously important and valuable part of the game, but I want them to be triggered by poor player choices rather than an event they have no control over.

One of the valuable aspects of the Hit Point system is that it gives a buffer where you realise the situation is turning pear-shaped before it gets to out-of-control. Obviously, at the lowest levels, it is not much of a buffer! But it does provide some!

Of course, having a buffer makes little difference if the players cannot change their minds and retreat. As a DM, you want to enable retreating, not stop it. It makes for more interesting play where the players can make decisions.