On the knights of noromath server, many critical hit effects have been added based on inspiration from the old critical hit tables from AD&D.
Any extra effect from a critical hit can be negated if the recipient of the attack succeeds a save vs. death. The DC of this save is 10+ 1/2 the hit dice of the attacker, plus the attacker's strength modifier.
When the recipient of a critical hit fails their saving throw vs. death, roll 1d20 to determine the severity of the effects. Half the base attack bonus of the attacker is added to the roll, as well as any bonuses from weapon material, weapon size, and the weapon damage type.
Severity Score Injury Level & Effects 1-3 No additional effect 4-8 Grazed; minor attack decrease, minor spell failure, additional "grazed" effects based on weapon damage type 9-14 Struck; moderate attack decrease, moderate spell failure, additional "struck" effects based on weapon damage type 15-18 Injured; major attack decrease, major spell failure, additional "injured" effects based on weapon damage type 19+ Instant Death
If the attack was a haste action or creature attack, it is treated as a medium-size weapon and is assumed to use slashing/piercing or bludgeoning/piercing damage tables. It still adds 1/2 the base attack modifier of the creature. These attacks are not modified by special material types.
Weapon size bonuses represent the ease of causing a more severe critical strike based on the size of the weapon. Small and tiny weapons receive a +2 bonus, medium weapons receive a +4 bonus, and large weapons and heavy crossbows receive a +6 bonus to their critical severity.
Special materials and equipment worn by a character have very real impacts upon the severity felt by critical hits. The armor, bracers, helmet, and shield slots may all benefit from being forged or made from materials like adamantine, dragonhide, mithril, steel, or tough animal hides.
Adamantine: Weapons, armors, bracers, helmets, and shields made of this metal have a profound effect upon critical hit severity.
Mithril and Steel: Weapons, armors, bracers, helmets, and shields made of these metals have a significant effect upon critical hit severity.
Dragonhide and Tough Animal Hide: Light armors, Hoods, and Gloves made of these materials have a significant effect upon critical hit severity.
Additional effects from critical hits include attack penalty, bleeding, extra damage, instant death, knock down, move penalties, spell failure, and stunning.
Attack Penalty: Attack penalties range from -2, to -4, to -6 and can be restored by rest or by any spell or effect that removes such.
Bleeding: Attacks that open up a bleed cause an additional 10% damage to the recipient of the attack.
Extra Damage: Attacks that do extra damage are considered to be tripled, effectively doing 50% more damage than the original critical hit.
Instant Death: The recipient of the attack dies instantly.
Knock Down: The recipient of the attack is knocked prone for 1 round.
Move Penalties: Movement penalties represent lasting injuries that hamper or impair movement almost entirely. These are removed via resting.
Spell failure: Any attack penalty is implied to carry a corresponding spell failure chance with it. This penalty is removed by resting.
Stunning: A stunning effect either lasts for 1 combat round or 1d4 combat rounds if the effect is of struck or injury level.