The presence of negative hit points differs depending upon whether or not the subject possesses an experience level. Those without a experience level are designated below as 'creatures'; those with experience levels are designated 'combatants.'
Combatants Without an Experience LevelFor creatures that do not possess experience levels, death occurs at negative 4. This means that at -1, -2 and -3, the creature is still alive, though in increasing levels of distress. During combat, at -1 hit points, a creature will be panicked and will turn berserk if that is a part of their nature (bears, boars, norkers, etcetera). As well, at -1 hit points, the creature's strength, intelligence and wisdom (treat wisdom as equal to intelligence), as well as other stats if they are known, should be assessed at 30% below normal (treat as 7/10ths). Most importantly, a creature's action points (AP) are also lowered!
If a creature had an wisdom of 4 normally, at -1 hit point the creature's wisdom would be rated at 2.8.
At -2 hit points, ability statistics are reduced further (4/10ths of normal), so that the same creature would have a wisdom of 1.6. This will challenge the creature's consciousness, so that a wisdom check must be made based on its present wisdom. The creature would need to roll a 1 on a d20 to remain conscious.
At -3 hit points, ability statistics are reduced still further (1/10th of normal). As before, creatures must make a wisdom check if reduced to -3 hit points - but unless the creature has 10 or more wisdom normally, a wisdom check at -3 hit points will automatically fail and the creature will fall unconscious.
Note that if a creature has been reduced to -2 hit points, and makes its wisdom check and is subsequently reduced to -3 by damage, another wisdom check must be made.
Combatants with Experience LevelsOnce a combatant has obtained sufficient experience to obtain first level (impossible for any combatant with less than a 3 intelligence), that same combatant will not be killed unless they are reduced to -10 hit points. Generally, reducing a leveled combatant to less than zero hit points works much the same as the description above - except that each negative hit point below zero reduces the combatant's ability stats and AP by 10%. Thus, in the example shown, the combatant's 18 wisdom drops by 1.8 as it drops lower in hit points.
damage the combatant has taken.
Once a combatant is lowered to -4 hit points, however, then a wisdom check must be made, based on the combatant's present wisdom. Like with creatures, combatants must also make a wisdom check every time they are struck for damage reducing them to -4 hit points or less. Moreover, once a combatant has been rendered unconscious, they must make a check against their wisdom so long as they remain below zero hit points, even if they are healed!
For example, Albert, who possesses an 18 wisdom, is struck for 5 damage and lowered to -4 hit points. He rolls a 9 on a d20, so he remains conscious - but as he is armored with chain mail and encumbered, at -4 hit points his AP has been reduced to 1 (fractions are ignored). Therefore, the most he can do is move away or attempt to attack, knowing he will not have a chance to hit until the following round (assuming action points can be spread over two rounds). Albert chooses to fall back one hex - but then he is hit again for 3 damage, lowering him to -7. Now he must make another wisdom check. He rolls a 6, one above what he needs, and falls unconscious.
As he does, however, Bala casts cure light wounds and touches Albert, restoring five of his lost hit points, bringing him to -2 hit points. Because his status has been altered, Albert makes another wisdom check. He needs a 14 to regain consciousness, but unfortunately for Albert, he rolls a 16 and remains unconscious. Resolved, Bala retrieves a healing salve and feeds that to Albert. Thankfully this gives him 2 more hit points, bringing his total to zero. Once Albert is at zero hit points, he automatically awakens without needing to make a wisdom check.
Partial Action PointsThe DM may, reasonably, allow a combatant some ability to perform part of an action point depending upon what action is chosen. In the example above, when Albert is lowered to -7 hit points, even if he had succeeded he would only possess 0.9 action points. This should still be enough to express three spoken words instead of four, or perhaps drink a lesser part of a healing potion, getting a fifth of it inside him each round until the whole potion is drunk.
See Attacking in Combat