Like AD&D 1st Edition, there are no real rules for Critical Hits and Fumbles (beyond the original AD&D Double Damage rule), these rules have been 'imported' directly from my original D&D/AD&D Campaigns (all those years ago LoL).
-10 HP & Death
Normally when a Character reaches -10 Hit points he dies immediately.
This version is ‘softer and bouncier’ . . . .
When a Character reaches Zero HP he is battered and exhausted, at Deaths Door – but not quite out.
However he does have a slight reserve left. He now resorts to just his Con (Normal folks in my games don’t Have HP just an average Con Score of around 8-9), if he continues to take Damage and he is reduced to half Con (or less) he must IMMEDIATELY make a Constitution Saving Throw – the difficulty is ALWAYS 18 regardless whether the Characters Constitution Score is Primary, Secondary, or Tertiary.
If it is failed, the Character passes out. If it is passed, the Character stays Conscious – and must continue making Constitution Saves every Round until he is healed and passes the Consciousness barrier (more on Healing later).
I the Character continues taking Damage, and reaches Zero Con he must make another immediate Constitution Save (again Difficulty 18 regardless) and if failed the Character dies. If passed he must continue making Con Saves each round. However now the difficulty of the Save increases by 2 each time he passes (the poor fellow is dying after all) until the he
receives Healing or finally expires.
Death Saves Table
Initial Save vs. Death – 18+
After1 additional Round – 20+
After 2 additional Rounds – 22+
After 3 additional Rounds – 24+
After 4 additional Rounds – 26+, and so on . . . . .
Critical Successes & Fumbles
Non-Combat Criticals & Fumbles
A Natural ‘20’ is a Critical Success, & a Natural ‘1-2’ is a Fumble when testing off a Secondary Attribute.
A Natural ’19-20’ is a Critical Success, & a Natural ‘1’ is a Fumble when testing off a Primary Attribute.
Combat Criticals & Fumbles
A Natural ‘20’ is a Critical Success, and a roll of ‘1-2’ is a Fumble – when rolling to attack in Combat, this applies to Clerics, Druids, Wizards, & Illusionists. They have limited skill in combat and are less likely to deliver a crushing blow.
An unmodified roll of ‘19-20’ is a Critical Success, whilst a Natural ‘1’ is a Fumble – when rolling to attack in Combat, this applies to Fighters, Barbarians, Knights, Rogues, Rangers, & Assassins.
If a Critical Success is rolled to hit, roll again – if another Critical Success is rolled the Damage is applied to the Characters (or Monsters) Constitution directly, just as if the target had no remaining Hit Points (see “-10 HP & Death” for further information). If no further Critical Successes rolled (or it the Hit is against a Zero Level NPC with not Hit Points) the Damage is Doubled instead.
If the Roll is a Fumble, it is an embarrassing accident and the Weapon is dropped. Again a further D20 is rolled, if another Fumble is rolled the Weapon not only dropped but damaged in some way. This (of course) only applies to Magical Weapons with no Intelligence and Non-Magical Weapons.