HOUSE RULES
Critical Hits
Any natural result of a '20' on a to-hit roll is a critical hit, unless the attack can only succeed of a roll of '20'; in which case a second to-hit roll is made and a second natural result of a '20' is a critical hit. Additionally, any to-hit roll that exceeds the target's AC by 10 or more is also a critical hit.
Critical hits inflict maximum damage for the weapon used plus the attacker's Strength modifier.
Level Drains (Energy Drains, Negative Levels, Etc.)
The level-draining abilities of certain undead monsters (spectres, vampires, wights and wraiths) will be replaced by Constitution drains, as this better represents a character's life-force. Constitution drains subtract a number of d4s from the character's Constitution equal to the number of levels the undead would otherwise drain. A successful save against Energy Drain reduces this loss to 1 point per die. If a character's Constitution is drained to '0' or below, the character dies and later returns from the dead as detailed in the draining monster's description.
Constitution loss is permanent, but can be restored via clerical 'Restoration' spells and other magical means.
Missile Weapons
Missile weapons damage rolls are open-ended (roll the damage die, if you roll the maximum score ['6' on a d6, for example], roll again adding the results together; as long as you roll the maximum score on the damage die continue to roll again and add the results together).
Critical hits, see above, are considered to be a 'natural roll' of the weapon's damage die's maximum score.
Spell Casting in Combat
Spellcasters can cast during combat. However, if they are hit before they can release their spell (i.e. earlier in the round, or at the moment of casting), they must make a concentration check or lose the spell. Wizards and illusionists use INT for their checks, whereas clerics and druids use WIS (the idea here is that the PC must keep his/her 'wits' and utter the enchantment, despite being hit).
The concentration roll = 1d20 + attribute modifier + PC level. The result of the roll must beat 12 (since the attribute is a prime) + damage taken, or the spell is lost.