Update: some of the material contained in this post is no longer accurate to the way that I run my game. An updated version of the rules proposed below may be found on the wiki, on this page. The original post is unchanged.
I haven't felt very healthy for quite a few days now, which explains the drop off in posting. But a house rule of mine came up during the online campaign, which I discussed briefly on the campaign blog ... but it makes sense to discuss it a little more fully here. And as I feel a bit better tonight, here goes.
The concept of negative hit points did not come from me. Honestly, I don't know where it came from, something I read in a games store no doubt, a long time ago. I'm always up for a good idea, however, and so I co-opted it. I have no idea what the original rules were. These are my rules.
I liked negative hit points for a couple of reasons. They gave a kind of safety buffer to low level characters, keeping them alive, while at the same time telling a character of any level that it was time to slow down and get the hell out of the combat. Strangely, when I used to play without that cushion, a player would go on fighting with two hit points even though he'd die at -1 (zero always meant unconscious). With the new method, if a player drops below zero, it will get across the message that it's time to go.
It works like this. A zero-level human, or an ordinary humanoid of any race, still dies at -1 hit points, and still falls unconscious at 0. However, a leveled individual of any race does not die until they reach -10 hp. This means that they can persist until they reach -9.
However, there are certain effects which begin to manifest once the player drops below zero hit points. For each point below zero, the player's stats are lowered by 10%. Thus, at -1 hp, the player has 90% of their ability stats, at -2 hp, 80%, and so on. All stats are affected equally and in the same proportion. This has some interesting applications where it comes to players.
Intelligence is the critical stat. As long as the character's intelligence remains above 3, the character may continue to function. This means that for someone with an intelligence of 10, they are still able to think their way through most things as long as they have -7 or more hit points. When a character drops below 3 intelligence, they are a vegetable.
Moreover, once a character begins to drop in stats below what they need for minimum class requirements, they lose the benefits that come with their character classes. Monks cannot open hand, paladins lose their 10' radius of protection, spellcasters lose their spells, fighters drop onto the zero level fighting table and so on. Usually these things don't become a problem unless the player drops below -3 hp, but in the case of paladins, illusionists or monks, who need 15 to 17 pt. stats, the effect is usually felt at -1 hp.
Even a small drop will be felt. A fighter with an 18/00 strength at zero hit points will find his or her power affected rather quickly. As I consider each 20% of strength bonus to be equal to one point of strength (it has always been a problematic system), the fighter's strength is considered for this rule to be equal to '23'. Each point below zero reduces strength by 2.3, so at -1 hp, the fighter would have a strength of 18/54; at -2 hp, a strength of 18/08; at -3 hp, a strength of 16.1 and so on. So comparative weakness takes effect very quickly.
A cleric with a 13 wisdom, reduced to -1 hp, would then have a wisdom of 11.7 ... the fraction is dropped and this is treated as an 11. While the cleric is still a cleric (minimum wisdom must be 9) and can still throw spells, now the cleric has a 10% spell failure chance according to his or her present wisdom.
Similarly, thieves lose dexterity bonus for thieving abilities, while mages lose their chance of knowing spells which are in their spellbooks. And all characters begin to accrue negative modifiers on their attacks and armor class as their strength and dexterity fall below 7 points. And yes, as charisma drops, so do charisma modifiers on hirelings and men-at-arms.
There is one more unlikely effect that wouldn't occur often, but would happen with certain characters. I play with the rules where a player cannot be a certain character if the stat is below six. Only mages may have a strength below 6, only fighters can have an intelligence below 6, only thieves can have a wisdom below 6 and so on.
If a character has chosen to have a cleric with an strength of 7, they are going to be heartily surprised to find that when their strength drops below 6 (at -2 hp), they are no longer a cleric, even if their wisdom still remains well above 9 ... since with a strength of 5.6, the only thing they can be is a mage - which they are not.
It is an interesting house rule, and has served well to instigate interesting game-playing effects among players once they drop below zero hit points and they must play weaker, stupider, uglier characters. The fun really begins when virtually every member of the party is below zero to some degree - a staggering, half-zombified mob just trying to get away while yet keeping their senses more or less intact.
For those who have trouble with math: