Certain spells and monster attacks put targets into a helpless state, including hold person, sleep and ghoul paralysis. Helpless targets are described as “conscious and able to breathe, but cannot move, act, or speak in any way.” Targets entangled in a web or by vines are not helpless. Likewise, targets in a wrestling hold are not helpless.
Helpless targets may be killed instantly with a melee blow, such as slitting the throat. The attacker makes no attack roll. The target instantly goes to zero hit points and is dead. No extra Death Save is made. In other respects, this is a normal attack. It ends the attackers turn. It does not trigger a parting shot.
Only living creatures with vital anatomy may be instantly killed when helpless. Those without vital anatomy are hit automatically for maximum damage. For example, a black pudding paralyzed by a ghoul has no vital organ to destroy, but a fighter with a long sword could do 8 points of damage automatically every round.
Attacks with ranged weapons on helpless targets do not hit automatically. The attacker must roll to hit with all normal considerations of range, cover and other bonuses or penalties. If hit, the target takes maximum damage. Add a +2 attack bonus for being equivalent to attacking from behind. Compute the target’s AC without the influence of DEX or a shield. Preserve AC bonuses from magical armor, rings of protection or similar items. (Yes, if the target has a negative DEX bonus, they are harder to hit when standing still.) If the target has a +1 shield, AC goes down by 2 (one for the shield and one for the magic bonus).
Alternatively, an attacker may choose to make a Called Shot to an unarmored location of the target, in which case a successful hit is treated as a melee blow described above. For example, an orc shooting an arrow at the unprotected neck of the fighter in chainmail suffers a -4 penalty to attack. The target still suffers an AC penalty as described above. The attacker still gets the +2 attack bonus, netting out to -2.
Related Material
This topic was discussed in the BFRPG forums: How do you handle paralysis?
If it is a ranged attack, I usually treat them as an immobile object (no dex mod, AC based on armor type due to trying to find the weak point in the armor) then the variable damage. Think of them being a dart board and the better the damage roll, the closer to the bullseye the shot gets. Now with melee, it is just an autokill as there is no rush, no resistance or fighting back. The character would just walk up to the npc and slit their throat.
Metroknight
To which Solomoriah (BFRPG author) replied, “Metro has the best take on this IMO.“
The D&D Rules Cyclopedia includes the following regarding the effects of the sleep spell: “Characters can kill a sleeping victim with a single blow of any edged weapon, regardless of the creature’s hit points.“
AD&D states about the sleep spell: “Note that sleeping creatures can be slain automatically at a rote of 1 per slayer per melee round.“