Aimed shots to the head, torso, or limb are ordinary attacks, no special modifier, standard 1d6 damage on a hit. Really small targets (eye, finger, heart) are treated as AC 0. The character actually makes several feints, near hits, and grazing blows in the round, exactly as normal, but if the d6 roll is 5+, the body part is "broken" or "deep cut" or "burn", whatever is appropriate. On 7+ or a repeated wound to the same area, upgrade to "very" or "bad(ly)" or some other comparative in front of the wound type. On 9+, the damage is total. However, some creatures do not have a heart, or can only be slain by smashing a gem on their forehead, or some other special rule.
Wound levels mean the wounded limb or organ is useless for a certain time:
- Wounded: useless until bandaged (-1 or half move after being bandaged)
- Badly Wounded: useless until healed
- Crippled/Severed/Lost: permanently useless. For head or vital organs, this equates to death.
Anything that modifies the damage roll modifies the wound test as well, which is the main way to severe heads or limbs with a single blow. If the damage roll is 2d6-1 or more, the modifier is applied to the highest die roll only, but each die result is treated as a separate test for a wound; multiple successes mean more serious wounds as well. Giants are very dangerous!
The opponent's Con bonus modifies the target number. A character with -1 hp per hit die can be wounded on a 4+, while those with +1 per die are only wounded on a 6+.
I'm debating whether armor should make aimed attacks ineffective until an attacker either find a vulnerable point or damages the armor. Maybe leather armor blocks critical hits from blunt weapons, and metal armor blocks all critical hits, which would explain why you might want to take that AC 0 aimed blow for the eyeslits of an opponent in plate armor.
Within this context, you can make up other targets and apply the same wound system. Thus, a wraith draining the life force from an opponent could be handled as "aimed blow towards the victim's soul"; draining isn't automatic on a hit, but goes much more quickly. But on the other hand, I don't think standard level loss is that bad as long as you don't drain experience points as well, which seems to be the standard interpretation from AD&D. If you just drain levels but keep xp the same, then a character can regain one level after each successful adventure, which means that severe level drain doesn't last as long.