Helpless Defenders
A helpless defender is any creature that is attacked while being unable to defend itself, because it is asleep, charmed or paralysed. Many an assassin will approach a sleeping victim through stealth; this is often the easiest means of killing a dangerous opponent. Because these opponents are not rated "combatants," experience is not awarded for the damage done to helpless defenders.
Causing Damage
When attacking a helpless defender, the attacker does not roll to hit. Instead, it is assumed the hit occurs, and that the damage done is 10-60% of the victim's maximum hit points above zero, each combat round. In the case of sleeping victims, the first hit will result in the victim awaking; but if the damage done is sufficient to stun on the first blow, there is little chance the victim will ever receive an opportunity to fight back, unless aided by an ally.
Assassins or thieves cause an addition bonus 10% per blow against helpless defenders, or 20-70% of the victim's hit points. Both assassins and thieves may attempt to assassinate or backstab the opponent; but if this fails, the damage that occurs is still a minimum of 20-70%.
Example
WORKING...
For example, a creature with 20 hit points is unconscious when attacked by a non-assassin. The attacker will roll a d6 x 10% x 20, or 2 hit points per pip appearing on the die. In all probability stunning the creature outright. However, since such an attack will wake a sleeping creature, or potentially ruin a paralyzation spell, most defenders will no longer be helpless and will have to be hit thereafter (even if they are stunned, they are still rated according to their armor class!).
See Attacking in Combat