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Except where noted, the rules are assumed to be the same as in the PHB and DMG. These are based on the Grim n Gritty rules by Kenneth S Hood. I reference some of the specific changes in boxes like this one.
Hit points are considerably more stable in grim Europ; you start with a number of hit points equal to your CON x2 (modified for Size), and they rise only very slowly thereafter.
Base hit points are then modified sharply by size. The below multipliers apply AFTER hit points gained per level, so Large characters will shoot up in hit points compared to their Small brethren.
Fine x1/16 Large x2 Diminutive x1/8 Huge x4 Tiny x1/4 Gargantuan x8 Small x1/2 Colossal x16 Medium x1
If you lose half your current hit points from a single attack, you are clobbered. You may only take a partial action during your following round.
These rules are somewhat different from Ken Hood's injury rules; I was looking for something a little simpler.
When you are at or below half your total hit points, you are mildly incapacitated. You are at -4 on all attack rolls, ability & skill checks, and saving throws. You also can not run or charge.
When you are at 0 hit points or below, you are completely incapacitated. You collapse and are barely conscious (or not conscious at all).
When you are at -10 hit points, multiplied by your size modifier, you are dead.
Characters recover 1 hit point + CON bonus per week of rest. Successful long-term care with the Heal skill increases this by +1 hit point per week, or +2 hit points per week for complete rest. This is multiplied by your Size modifier for hit points.
Armor no longer adds to AC (although shields still do). Instead, armor adds to your Damage Resistance (DR), as described in the next section.
In Ken Hood's original rules, AC was renamed Defense and Damage Resistance was renamed Protection (this was presumably done for clarity). I prefer using the standard d20 terms for these effects, however, and do so throughout this text.
AC starts equal to 10, and is modified by your DEX bonus, any BAC you purchase with CP, your size (see chart below), and various circumstance bonuses (such as a shield).
Size AC Mod Fine +12 Diminutive +9 Tiny +6 Small +3 Medium +0 Large -3 Huge -6 Gargantuan -9 Colossal -12
The AC Mod for size may seem unsually high, but these numbers result in a more realistic chance of hitting someone who is twice your size in every dimension than the small +/- 1 of the PHB.
Shields provide their normal amount of AC bonus.
You lose your class-based bonus whenever you are unprepared, ambushed, stunned, immobilized and the like. You do not lose your class AC bonus when attacked by an invisible opponent.
Armor limits your DEX bonus, and thus may reduce your AC if you are wearing heavy enough armor.
Ken Hood's rules call for an additional penalty to AC for wearing armor. Although this seems intuitively true, armor was not really restrictive in terms of movement, as long as you were strong enough to carry its weight. I prefer to allow the encumbrance rules affect your AC directly.
If the spell actually makes you harder to hit (such as Shield), it benefits AC. If it protects you from harm in a manner similar to armor (Mage Armor), then it provides DR instead.
Armor (as well as a leathery hide, scales and similar things) provides Damage Resistance. To calculate DR for armor, convert its AC under the standard d20 system into DR. Thus, a chain shirt normally gives +4 AC; in Europ, the chain shirt will provide DR 4/-.
Magical armor adds its bonus to the DR of the armor. Armor DR and natural DR (such as from the Leatherneck feat for the goblin-tainted) stack with each other.