My idea is this: that BACKSTAB be made a Thief Guild skill — maybe a Thief-Guild Only Command, period — as a representation of the dishonorable, shady fighting style that the less-armored ruffians of the world employ to try to level the playing field against heavily armored protectors of the law. Backstabbing someone would NOT require a player to be hidden first, but WOULD use the 'hide' and 'sneak' skill, along with the Dexterity stat, as supplements to its success. For the target, having a weapon prepared would grant a higher chance for the attack to fail, as well as having a higher WISDOM stat of the attacker, to simulate seeing their underhanded efforts. A backstab would, by default, do no increased damage but would bypass armor if successful and, depending on the location targeted, add a particular affect to the victim to give the attacker an advantage. Defense values up to the attacker's Backstab Skill Level would be ignored. Someone who has been backstabbed recently would be immune to backstabs for a long period of time, but attempting to backstab someone who has this immunity would throw-back a coded failure that does not alert the victim that an attempt was made. ("They are too alert for this techinique to work.") A failed backstab would initiate combat, apply the 'alert' affect to the victim for a reduced time period, and impose a penalty to the initiator's defenses, as they are put off-balance by their failure, along with the attack itself auto-failing and doing no damage. Attempting to backstab non-human NPCs would result in an auto-failure: this skill is reserved for humans. Backstabbing would be ICly seen as very much a thieve's action, and a failure would still be an obvious attempt at it.
tl;dr: Backstab becomes Thief Guild Skill, uses Dagger Skill, Dexterity, Sneak, Hide vs. Wisdom, Defenses with Bonus for Weapon Drawn for success. A successful backstab deals a blow that applies a certain type of affect depending on type of backstab used and ignores armor in the given location. Backstab Skill affects types of backstabs available, ignores victim's defenses equal to the backstab skill, and at higher ranks ignores the 'drawn' weapon of the victim. On success or failure, combat is initiated and victim is given an 'alert' affect making them immune to backstabs for a period of time. On a failure, a large penalty to defense on the initiator and no damage is dealt. Backstab cannot be used against non-human targets.
To incorporate Prisca's cool idea, AMBUSH would be made its own command or skill, drawing on the Sneak and Hide skills (which are not guild skills) for its success. An AMBUSH involves preparing a moment for an advantageous attack on a target, and would have a higher than average critical-strike chance (perhaps ignoring a percentage of a victim's defense skills?) and place the combatants at the ideal range for the attacker, no matter their coded locations beforehand (making it especially potent against PCs). Attempting to ambush in public locations would be less succesful. While it would initiate combat, it would not count as the initiator's action in the first combat round; in effect a 'surprise round'. A failed ambush would deliver a normal attack but leave the attacker off-balance with slightly lowered defenses. Ambush would not ICly been seen as a strictly thief-type attack, and would be useable with every type of weapon or method of combat, including mattack.
tl;dr: Ambush draws on hide + sneak skills for success and, if successful, places the attacker at the ideal range for their weapon vs the target and deals increased critical strikes. It counts as a 'surprise round' in combat, initiating combat but not using the attacker's action for the round. Failure begins combat as normal but imposes a small defense penalty.