One of the first D&D houserules I encountered was the “crit to kill” rule: if you roll a natural 20 on an attack, roll another d20. On a second 20, the guy dies.
In Jon Peterson’s Playing at the World, I found a reference to a 1975 ancestor of that houserule, published in 1975 by Gary Switzer, the guy who wrote the first version of the Thief class. Peterson says, “On each melee swing, the attacker rolls an additional d20, which if it scores a ‘0’ (bearing in mind that early d20s had two 0’s), results in a critical. It is the main attack die which determines whether this is a critical hit or a trip—if the attack roll succeeds, then a hit has occurred, otherwise it is a trip.”
That extra die roll never made it into D&D canon, but 3e introduced the idea of “confirming crits:” rolling to see if your natural 20 was really a crit or not. (I think it introduced it. As I’ve mentioned before, my weakness is 2nd edition rules.)
I don’t like any of these rules very much in actual play, but not for game balance reasons. Sure, an insta-kill on 1 in 400 attacks adds wackiness and mitigates against PC survivability, and confirming crits only helps in bizarre corner cases where goblins crit on 100% of hits against dragons. The reason I don’t like them, though, is because they add anticlimax rolls.
In the “crit to kill” variant: You crit! Roll another d20. On a 1-19: Oh well, at least I got a crit. With the confirming a crit rule: I rolled a 20, but failed on the confirmation roll! Oh well, my crit didn’t happen.
4e’s solution was to have a crit always do max damage, and then throw some extra damage dice into the mix. That was not a bad solution: the extra dice always felt like bonus damage, even if you rolled poorly.
I COULD imagine bringing back the “crit to kill” rule in a modified form.
Recently, I’ve experimented with rolling d20s (and even d100s!) for special-effect damage. It’s fun! In my proposed variant, when you roll a crit, you don’t double or max your damage; you throw in a d20 along with all your other damage dice. If that die rolls a 20, you insta-kill. Otherwise, it just adds its damage to your hit (an average of 10 extra damage).
If the 1-in-400 chance of an insta-kill is too silly for you, you could instead make it an exploding d20 roll: on a crit-die roll of 20, you add 20 damage and roll again. Against all but high-level opponents, it will come to the same thing. However, this tweak gives 20th-level fighters some protection against 400 goblin archers.
My old 3.5 DM used the confirm roll, but if you rolled another crit on your confirm roll, you got a third roll. If the third roll was a crit, it was an instant kill. He didn’t require a natural 20, just a critical hit. Some of the 3.5 weapons have a wider crit range (a lot of the hacking weapons were 19-20, for example, and the rapier was 17-20, I think), plus there was the expanded crit roll feat, that doubled your range. Still, it didn’t come up that much.
In the final game, my great sword wielding fighter/mage with expanded crit feat did manage to autokill as aspect of Tiamat, but the party had already dumped a ton of damage on it. It made for a fairly dramatic ending to the fight. That said, if I had hit it earlier in the combat, the whole thing had the potential to be anticlimactic.
In the Labyrinth Lord game we’re playing, the DM uses a location table. Only a natural 20 is a critical. Then you roll again for location. Each of the locations have different outcomes. Some body parts have the potential for autokill (like the head, chest, neck) or limb severing. If you hit one of those, you then roll percentiles. If you can roll under the damage you dealt, it’s an automatic kill/severed limb. I don’t think any of us have pulled it off in over 36 sessions, though.
Here’s the chart, if anyone is interested —
https://docs.google.com/document/d/1htENtsqd2v-Zmu3arxVqf8LeAZWkbVi-vVcG-WCJobo/edit
The house rule I run with is “If you roll a natural 20, and you would have hit even without the ‘natural 20’s always hit’ rule, then it’s a crit.” It eliminates “crit or miss” weirdness without requiring an additional roll.
Our house rules were:
Natural 20 always hits and allows an extra roll. If the extra roll (fully modified) also hits, it’s a critical hit. Consult appropriate nasty charts of critical damage results. If the extra roll misses, roll double damage.
This was our final set of crit charts: http://www.pbegames.com/deadlyblows/
We have a variation. Each 20 adds +1 to the multiplier. I’ve seen some triple 20 instant kill rules before. Also, the double 20. Our group likes to put bad guys at a disadvantage and have fun. I don’t know that having even odds is ever really fun. (the 400 goblin rule)
The common house rule that my friends and I used during the 90s (with 2E) was: on a natural twenty you could either do full damage or double damage (before bonuses); player’s choice. It always added a fun little decision about whether to go safe (full) or risk a big payoff (double, with the potential of something like doubling a 1).