Pathfinder 1E Rules for Instant Death.

[Cadence]

20-20 instant death does have a long history though... and Gygax was decidedly not a fan due to the swinginess (see bottom).

Earliest I've seen for myself was part of Michael E. Mayeau's "The Dragon Crown" OD&D adventure that was run at Pacific Encounters Convention in 1978 and published the next year by Judges Guild (16 pages for $1.75). Might be the second module I ever got (after B2 in the boxed set). Gotta love a 1 in 8,000 chance of killing yourself on each attack.

After a roll of a natural '20' roll another d20:
1-14=No critical
15=Maximum damage
16=Damage roll x2
17=Maximum damage x2
18=Damage roll x attackers level
19=Maximum damage roll x attackers level
20=Instant death

After a roll of a natural '1' roll another d20:
7-20=Normal miss
6=Stumble, roll your dexterity or less on a d20 or fall. If you fall, each melee round you may attempt to rise by rolling your AC or less on a d10 (with special things about magic armor and getting help).
5=Weapon breaks. If a magic weapon, roll 2d6 and add your weapons plusses to the roll, if your total is 7 or less your magic weapon broke.
4=Hit nearest ally for 1/2 damage
3=Hit yourself for 1/2 damage
2=Possible critical hit on nearest ally.
1=Possible critical hit on yourself.

"Empire of the Petal Throne" had one in 1975 that was twice as instantly deadly (courtesy of Wikipedia):

The 1975 role-playing game Empire of the Petal Throne introduced the concept of critical hits (though not the phrase). Using these rules a player who rolls a 20 on a 20 sided die does double the normal damage, and a 20 followed by a 19 or 20 counts as a killing blow: "this simulates the 'lucky hit' on a vital organ."

I've heard that name before, but either never new any details or forgot them all:
http://grognardia.blogspot.com/2009/01/retrospective-empire-of-petal-throne.html
http://www.tekumel.com/about_productlist.html


The "Arduin Grimoire" in 1977 had a percentile table you got to roll on if you got a 20-20... but it wasn't always lethal.
http://jrients.blogspot.com/2007/08/great-random-charts-from-rpg-history.html


Beyond instant death, a cool page on the history of criticals in general apparently popped up just last weekend:
http://playingattheworld.blogspot.com/2013/09/the-first-critical-hits.html

Gygax wrote in Dragon #16 (July 1978) that "the 'critical hit' or 'double damage' on a 'to hit' die roll of 20 is particularly offensive to the precepts of D&D." When critical hits (or fumbles) are played, as he puts it, "the whole game system is perverted, and the game possibly ruined" by the precipitous deaths of powerful monsters or player characters. This text anticipates the blanket dismissal that would show up in the Dungeon Masters Guide the following year of "such rules as double damage and critical hits" (pg.61).

 

[Angrydad]

My group adopted a method where a critical threat on a confirmation roll means you roll again, and if it is a hit, the damage multiplier goes up by 1. This can keep continuing if the player rolls threats on confirmations for a while. The most we ever have had though was 3 in a row for a scythe wielding fighter (that was a lot of damage). I like the idea of multiple natural 20s causing instant death though.

 

[Matthias]

I would only have house rules for instant death if there was some way for a PC to pay to get out of it, such as with Action Points, Hero Points, or whatever.

I generally dislike instant death rules for the same reasons instnat death spells and instant death traps are bad, because it's hard for PCs to come back from the dead at low levels, and trivial to do at high levels--so for most of the duration of a campaign, they are just 'unfun' parts of the game that might or might not happen, but when the rules do get invoked they do not really make the game enjoyable for the players.

Of course, without any actual risk of PC death, the game also ceases to be fun, so you can't simply design the game to make it impossible for a PC to die and still expect the player to realistically portray a character with an intact survival instinct. You have to allow for the possibility of death to maintain a measure of tension in any encounter, no matter how trivial the chance of ultimate failure may seem.

But there can be an implicit risk of PC death without automatically ruining the game. Generally the players will balk at seemingly nonsensical death, but most reasonable players will accept a death as the result of terrible luck on the dice or bad player decisions. The problem is that instant death rules as usually adopted are still too arbitrary and are perceived to give too much power to bad luck for the players to not get upset when the instant death does occur. There may be no true statistical difference between a really rare 1/8000 chance of a triple-natural-20 instant-kill, and a string of total misses for the player characters and some really good crits and high damage rolls for the monsters that in combination lead to the death of a PC. But the triple nat-20 death will still be thought of as unfair and arbitrary death, however rare, compared to the series of bad-luck rolls experienced otherwise.



Anyway, most instant death rules as designed tend to apply equally to all classes and levels, and in practice become more deadly and permanent for a low-level character than for a high-level one. Because most campaigns begin at low-level, their unintendedly severe penalty will be experienced by more low-level than high-level characters, further tarnishing the system's reputation. And the all high-level characters were once characters who were low level but managed not to get killed off by things like instant-death effects.

 

[abellius]

That is a house rule. It is not an official rule of the Pathfinder RPG, and I should say that even without knowing Pathfinder well I knew that was a house rule on the grounds that the designers of the Pathfinder RPG are usually quite competent.

I would strongly advice against adopting such a house rule, as it is far more detrimental to the PC's than it is to the NPC's (as for that matter are all critical hits). The problem is that the PC's are expected to dispatch 100's or 1000's of NPC's over the course of their career. So an 'instant death' of an NPC as a result of good luck by a PC doesn't really change much for the PC's who will mow through most enemies anyway. So, you saved a round or two of fighting by beating an NPC early.

But the NPC's usually need dispatch the PC party only once. PC's are subject to 100's if not 1000's of attacks in their career. If there is a 1:8000 chance an attack will kill them, then it's pretty much guaranteed that PC's will be instantly beheaded no save. If this occurs at an inopportune moment, it's permanent PC death no save allowed. Worse, if it occurs in an actually tense combat early on, it's possibly going to start a snowball that leads to a TPK.

I think that it is more than enough that a PC critical could lead to massive damage and a 'Save or Die' situation , and even that is an optional rule in Pathfinder.

I agree completely. Killing PCs in this fashion usually gets players upset and causes problems.