Any constructive comments are appreciated, such as how to improve this, or if something simply doesn't make sense and needs clarification. Just because it works in my head doesn't mean the words on paper translated the concept I envision correctly.
I have made some edits, based on comments and things that I thought needed clarifying otherwise.
Alternate Death and Dying Rules
These alternate rules apply to humanoids and goblinoids, and due to complexity over standard rules are reserved for only PCs, important NPCs, and main villains with other humanoid mooks and cannon fodder enemies using standard rules. Outsiders, elementals, magical beasts, aberrations, undead, plants, animals, vermin, and constructs use their standard D&D rules in regards to death and dying.
A creature has a number of negative hit points equal to its CON modifier * level, unless this total is less than 9. A creature with at least 1 CON has a minimum of 9 negative hit points. For the purposes of these rules, 0 HP is treated as if it is a negative value but does not count as part of the negative HP total. (i.e., if a character is supposed to have a max of 15 negative HP their hp can go to -15 and still be alive.)
Health states are recognized as the following:
Normal: 1 HP to total HP
Injured: (1/2 negative HP total, rounded down) to 0 HP
Crippled: max negative HP to (1/2 negative HP total, rounded down, minus 1)
Dead: below max negative HP
10th level fighter with 16 CON and average HP
total HP: 89
injured: -15 to 0
crippled: -30 to -16
1st level wizard with 12 CON:
total HP: 5
injured: -4 to 0
crippled: -9 to -5
Health states and their meaning:
Normal: No penalties. As a character's HP approaches 0, they become visibly more tired and suffer superficial scrapes and bruises, but nothing debilitating. Damage to the positive HP isn't "real" damage, it's an indication of the character becoming winded and less able to guard against real damage. A higher level character has more HP simply because they're better adept and experienced at turning what could be a mortal blow to a lower level character into a glancing one that still hurts but isn't life-threatening.
Injured: -1 to attack, damage, skill checks, and saves. A standard OR move action can be made without issue, but any action that requires a full round (double move, charge, full attack, etc.) causes the character to suffer a loss of 1 HP due to aggravating their wounds. At 0 HP and less a character has suffered real damage, and has cuts, burns, etc. that require medical attention or abstention from strenuous activity to heal naturally. As HP loss is entirely dependent on the character acting, there is no need to stabilize.
Crippled: -2 to attack, damage, skill checks, and saves, base speed is halved and cannot run, and loses DEX bonus to AC. A character in this state has sustained extreme injury and needs immediate medical attention. They are suffering from broken bones, internal bleeding, 3rd degree burns over much of the body, etc. Full round actions are impossible due to the broken state of the body, and the character loses the ability to effectively dodge blows. Pain makes concentration difficult, so casters must succeed at a Concentration check equal to their current negative HP in order to cast a spell. The character loses 1 HP per round regardless if action is taken; stabilization stops this loss (but will resume if any action is taken). Upon the completion of any standard or move action, the character must make a Fort or Will save (whichever base save is greater) against a DC equal to their current negative HP or fall unconscious. A character in the crippled state that has been stabilized will heal naturally as long as they receive a successful Heal check or are affected with a healing spell every 24 hours they remain in the crippled state. Once unconscious due to a save failure, the character remains unconscious until their HP is raised to at least injured status.
Dead: Simply put, dead. The body has sustained damage beyond that of normal magical or natural healing. However, the soul will not leave the body for a number of hours equal to the character's WIS+CON stats (not modifiers), representative of the strength of will and body holding the two together. During this period, a Raise Dead spell can be used. Gentle Repose extends the hours to days, so if a character with 12 WIS and 14 CON would normally have their soul bound to their body for 26 hours after death this spell extends this period to 26 days (minus any hours that have passed between the time of death and the casting of the spell). An alternate interpretation is that until the soul leaves the body is somewhere between coma and actual death and the correct kind of magical prodding (Raise Dead) can reawaken the character instead of it being a resurrection from actual death. From outward appearance, a soul-inhabited dead body appears to be actually dead (no breathing, no pulse, etc.), but it will not begin to decay until the soul leaves the body. Raise Dead requires a whole body, or at least the parts necessary to allow a creature to live on its own and can not restore missing pieces; if a body had been torn to shreds it will require a Resurrection spell to reform the body even if the soul is still present within the remains.
A character who has resigned themselves to death and has no intention of being raised (such as a Kelemvor cleric) may, at their option, chose to have their soul depart immediately or at at given time after death of less than or equal to their CON+WIS hours. This choice must be made at or before the moment of death, but not after. A character who choses to depart early can not be affected with Gentle Repose as they will leave at their designated time once it is set, and this spell effect will fail. (One caveat - if Gentle Repose is cast on a character who accepted their death, but was killed by CON drain with negative HP at the time of death and this effect also includes spawning undead from the corpse, it still stops further CON loss - see below in the spawning undead section.) If Raise Dead is attempted before a soul's designated departure time (whether it is the default time afforded them or a chosen time) and the soul still remains in the body the raising will be successful; the soul is completely unaware of its time between death and appearance in the Outer Planes and thus does not get a save. Resurrection offers a choice to return as the soul is aware of being called back; Raise Dead offers no choice.
The soul departs immediately from the body of a character that died of old age.
A soul with sufficient ties to the material plane (at DM discretion or for plot reasons) may become a ghost instead of making its way to the Outer Planes once it leaves the body. As with any undead, the ghost would have to be destroyed to raise the character back to life via Resurrection, whether by external forces or by its own choice.
Subdual damage is accrued as a positive number throughout a single combat instance. Once this total surpasses the character's current HP, they must make a Fort save against the amount of total accumulated subdual damage or pass out. If passed, they do not need to make another save until they accrue more subdual damage. This makes it unlikely for a mildly injured character to fall unconscious from a single punch to the face but not from an attack that does real damage.
Death and resurrection
While the soul is still bound to the body and the body is intact, Raise Dead can be used to return the corpse to the character's minimum HP and restore the soul to control of it; the raised character is unconscious and stabilized until natural healing or magical healing raises them to at least injured status. The soul does not have any recollection of the time between death and being raised. Raise Dead is essentially a healing spell that both restores the body to a minimum level of health to be considered alive and restores the soul's binding, but it can not reach across to the Outer Planes to call a soul back - the soul must still reside in the body. Level loss occurs due to the shock of the soul being reawakened from its stasis. Once the "grace period" has been passed on a corpse, the soul's attachment to the body decays to the point that it leaves for the Outer Planes and can only be restored by Resurrection or True Resurrection.
Constitution drain and spawning undead
When CON is drained both the positive HP and negative HP are affected. As long as CON remains above 0, negative HP can not be reduced to less than the range of 0 to -9. If CON reaches 0, the character is unconscious but retains all positive HP not afforded by their CON stat (i.e. the HP gained from rolling dice) minus any damage already sustained. The character loses 1 HP per round until their CON is restored to at least 1. If their HP reaches 0 while their CON is 0, they die, and the death rules above apply. In this fashion, an effect that reduces CON to 0 is not necessarily instant death (unless they have already sustained damage in excess of their remaining non-CON HP) but it is still fairly swift unless action is taken to restore the affected character's CON. Healing spells can be used to stave off death, but the character can not be stabilized as long as CON is 0, and they will continue to lose 1 HP per round until their CON is brought to at least 1.
If CON is drained to the point that the character's new minimum HP is higher than their current HP (if the character was at, say, -12 and their new CON stat makes their max negative HP -9), they die immediately.
If the CON drain is part of an attack or effect that spawns undead, the rules applying to the creation of such undead occur once the CON is 0 and HP is not a positive number (including 0). If a character died due to CON drain (but not due to being reduced to 0 CON) the corpse loses its remaining CON at the rate on one point per round until it reaches 0; this CON loss after death can be stopped by casting Gentle Repose on the corpse. When undead are spawned from a newly-dead corpse with 0 CON, the soul is forcefully torn from the body and only destruction of the undead and Resurrection spells can restore them to life.
Tracking CON loss after death if the CON drain comes from poison or disease is unnecessary; this only applies if the effect that caused the death also has an effect upon death. If the attack that causes the spawning of undead does not rely on CON drain, but simply death, the effect takes hold immediately after death regardless of CON stat.
Instant death spells
Spells that cause an instant death effect still do exactly that. The character is treated as if they had died from HP loss or CON drain. If a spell does normal damage and causes death at 0 HP (e.g. Disintegrate), death occurs if the character falls below their minimum HP instead of 0 HP from the attack but the effects surrounding the method of death remain the same.
Blood Knight [fighter]
Prereq: CON 16
Usable during one encounter / day, the character may treat injury categories as one level higher than they are for stat penalties (injured carries no penalty, crippled treated as injured), but they must make a Will or Fort save (whichever base save is higher) against a DC equal to their current negative HP value every round they remain in the negative to retain this benefit. A save failure reverts the character to normal injury behavior for the duration of the encounter. This feat's benefit may be invoked at any time during an encounter and simulates the ability to shrug off incredible injury and keep fighting. The character dies normally if they go below their max negative HP.
To the Bitter End [fighter]
Prereq: CON 14
A character with this feat does not need to make saves to remain conscious while in the crippled state. All other penalties still apply.
Masochistic Rage [general]
Prereq: rage ability, CON 14
Any character with the ability to rage immediately gains +4 to STR and +2 to Will saves while raging and in the crippled state. If the character also has the Blood Knight feat, being in the crippled HP range triggers this feat's benefit even if they are treating the penalties as injured.
Last Resort [metamagic]
Prereq: any metamagic feat, ability to cast arcane spells
Usable once per day, a wizard or sorcerer may apply any one metamagic feat they possess to any spell they have prepared without altering spell level (e.g., a maximized fireball cast via this feat would still use a 3rd level slot) as a standard action while in the crippled state. The Concentration check still applies, but a failed spell does not count against the single usage per day restriction on this feat. If the character also has the Blood Knight feat, being in the crippled HP range triggers this feat's benefit even if they are treating the penalties as injured.
Shoot Me Again I Ain't Dead Yet [general]
Prereq: CON 16
A character with this feat gains a bonus -10 HP below their total negative HP. When in this bonus negative HP range, the character is treated as crippled and unconscious (no save, immediate crippled unconsciousness even with other feats that alter injury categories or negate unconsciousness in the crippled state), but not dead. A character with 0 CON and this feat dies when their HP drops to below -10 instead of below 1.
New Weapon Enhancements
+4 enchantment, melee or ranged
A particularly nasty enhancement that makes a weapon perfectly suited for an assassin. Any creature killed with a Soul Rend enchanted weapon has their soul forcefully torn from the body upon death (the soul immediately proceeds to the Outer Planes), and only Resurrection or True Resurrection can restore them to life.