(no)TPK rules

It is difficult to say if TPK is something that you need to do, or not.
As it is very easy do obtain it, as a result, at least, if your player characters ain't escaped from Azkaban, there is no reason to write an article about that.
To avoid it is something completely different...

When on module creation process, my first choices would result in TPK. Oversized monsters, deadly traps, too numerous foes are as many reasons to kill the party. I want grand scenes and visual effects. But my players, as good as they can be, and as well as they know me, would prefer save their characters.

I have to pace myself and to make reasonable choices. But, most of the time, I keep visual effects.

Many years ago (and by that, I mean many, many) I found the outstanding works of Lenard Lakofka who's Tiny Hut article in Dragon magazine #48 literally turn this problem into gold.

At this time, this method for gameplay equilibrium would have needed hours of difficult math. Nowadays, a simple Excel sheet is enough. Despite my fellow dungeon masters who find this method "too scientific", I guess this simple objection for them will enlighten your curiosity. As usual, the principles are simple. The "problem" are the math. which, of course, with the right tool, are not.

1. Sum up all team experience points. Take only in account the minimum xp needed to reach their current level ie : 35 000 for a fighter player character of sixth level . Add ten percent to the amount. This gives you the "budget" you have to "spent" in your monsters.

2. To know the "Experience Point Value" of one monster, multiply his xp value by 50, plus 5 for every special power, plus 10 for every exceptional power. Those are given in page 85 of the DMG.

This value is the mass of monsters your party will have to defeat to face the "Greater enemy", aka the final boss. 

3.To calculate the value in "Experience Point Value" of the final boss, take the same you calculated for total monsters. ie : for an average party of six level seven character 165 000 xp, plus ten percent, equal a level nine magic user or a level height cleric. Do none hesitate to use unspent xp points to give primary class to you final boss. In my example the wizard 9, will have 46 000 unspent xp, let's say that he had a very interesting childhood, giving him six levels of fighter. Or seven levels of assassin, depending on your level of sadism. You're done.

Of course, this can be used for an undead final boss or what ever come across your insane but fertile imagination. Using the same math method than for your monsters.

This is empiric, as says in Lakofka explanations. It won't guaranty your party to survive, but it would surely push them to their limits.

Second part of the article is a try to extend this method to treasures. Personally, I find it less interesting.

Of course, this can be used for an undead final boss or what ever come across your insane but fertile imagination. Using the same math method than for your monsters.

This is empiric, as says in Lakofka explanations. It won't guaranty your party to survive, but it would surely push them to their limits.

Second part of the article is a try to extend this method to treasures. Personally, I find it less interesting.