[B9anders]

DF seems to prefer emulating the classic thieving backstab with Striking ST. I think there is a better way by using the rules in Martial Arts. The combination below can be bought as a 10-point package for your Thief/Assassin, allowing him to use it at full skill:

Lethal Assassin's Backstab (Hard)*
Default: Judo or Wrestling+4/Knife-7; cannot exceed Judo or Wrestling+4/Knife+0.
Prerequisite: Wrestling or Judo and Knife skill..
*As a combination with two rapid strikes, this technique costs 2 character points more to improve than standard Hard Techniques.

This is a special Combination designed for eliminating people as quickly as possible. You may only use this technique when making an impaling attack from behind with a reach C knife; you may take a step (before or after) if you wish.

Initiate the combination by making a Judo or Wrestling roll, at +4 to hit, to grapple the torso. If successful, follow-up by making two attack rolls against Knife-7! On each hit, you strike at the vitals for (x3 injury), doing an additional +4 damage (or +2/die, if better). A miss by 1 strikes at the torso. Successful or not, you have no defenses until your next turn.

If the victim is aware of your presence he can dodge or parry the grapple normally, though he is at -2 to parry from behind. He can not block. If the grapple succeeds, he defends at a -4 penalty to parry and -1 to dodge against the actual Backstab attacks and still can not block. Bear in mind penalties to parry for having to evade 3 attacks on one turn! If the victim is unaware of your presence thanks to successful use of Stealth, he gets no active defence!

On the victim's subsequent turn, he may attempt to free himself from the grapple (note that the attacker does not get the +5 bonus to maintain the grapple as it is only made with one hand), attempt a Wild Swing (p. B388) or one of the attack options listed under Pain and Breaking Free in Martial Arts (p115). Note, Unless the victim has Combat Reflexes, he is likely to be mentally stunned from surprise, for an additional -4 to his defense roll!

if the defender fails to break free of the grapple, the attacker can attempt another Backstab on the succeeding round without rolling to grapple. The victim can attempt a dodge at no penalty (the penalty for being grappled is cancelled by having backstabbed him previously) or a Parry at -3.

If the attacker misses with one of his attack rolls – or if his victim makes a successful active defense against it – the victim gets +3 on rolls to avoid the remaining attacks, provided he gets an active defense.

Backstabbing is a dirty trick. Rogues with the relevant perk get their bonus to all rolls in the manoeuvre on a first attempt. Backstabbers often also utilise a Reversed Grip for an additional +1 to damage.

A slightly less lethal, but more stealthy variant is also available:

Silent Assassin's Backstab (Hard)*
Default: Judo or Wrestling+1/Knife-7/; cannot exceed Judo or Wrestling+3/Knife+0.
Prerequisite: Wrestling or Judo.
*As a combination with two rapid strikes, this technique costs 2 character points more to improve than standard Hard Techniques.

When attempting the initial grapple, roll against Judo or Wrestling at only +1 to hit to grapple the face. If successful, the victim's mouth is covered and he cannot shout for help! The succeeding backstabs are only at +2 to damage (or +1/die, if better).

If the victim does not escape your grapple, you can follow up with additional Backstabs on subsequent turns for as long as the victim remains grappled. Attacks on these subsequent enjoy the same +4 damage bonus (or +2/die, if better) as the Lethal Assassin's Backstab!

Everything else is as per Lethal Assassin's Backstab described above.