Is there a way to make backstab blow with scimitar(normaly it isnt multiplied), by some editor or change in some file, i play as a dual fighter/thief and that is very anoing since scimitas are the best long sword in the game)

I can't really imagine how you could backstab someone with a scimitar in real world.

Easy , if u are strong(and my hief is , couse before he become thef he was fighter) enough then precisely holding a large sword is not big problem (and another point is that if u can with normal large sword than i dont know why u cant with scimitar which is ,IMHO, similar length.

In BG2 u could backstab even with big twohanded sword so why not with scimitar.(and as i say i love play with thief/assasin so i want him to be able to use best weapons in da game). So if so know how to use please tell me. THQ

some weapons that the game doesn't expect thieves to use can't backstab - exactly which scimitar(s) are you trying to backstab with?

In BG1 and I had previously thought BG2 as well, but possibly I am mistaken, you cannot backstab with any weapon a pureclassed thief cannot backstab with. As far as I know these weapons are hardcoded into the game, and not changeable. They are possibly marked with a certain flag, or maybe if you change a weapon's usability to include thieves, or maybe it is even in a 2da file. But I am pretty sure it's hardcoded. Wait for someone who knows more though, I'm hardly an expert on this...
-Brainless One

Hah what can be da biggest monster swaper ?? - that must be Dritzt scimitars which my thief wanna use. And talking about BG2 , i was playing it as a thief of course, and was running with "Holly Avanger +5" (if im right couse ive played polish version and the name was change to "Krasomir +5 " which i found rather funny and bit of lame)

A "Backstab" doesn't literally have to be a piercing stab from behind, just a surprise attack at vital areas when the victim isn't prepared. Otherwise, blunt weapons like Clubs and Staves could not be used to Backstab (some call this Backwhacking).

romeck, you have been misinformed: Thieves in BG2 cannot Backstab with 2-Handed Swords, unless there's some funky 2-Hander that can be used by Thieves (which there isn't).

Backstabbing with Scimitars in BG1 is iffy: According to BG1 rules, Thieves cannot use Scimitars. The Scimitars that are added into game when you install ToSC reflect this: They are not valid Backstab weapons for Fighter/Thieves (or, if you have BGTutu installed, Stalkers). However, there are a couple of Scimitars in the original installation of BG1. Since the classification of 'Scimitar' hadn't yet been written into the game, the default-BG1 Scimitars are instead classed as Long Swords: Thieves can use them (but Druids, I believe, cannot), and Backstab with them.

In BG2, the rules change a bit: All Scimitars are valid Thief weapons.

hmmm strange couse i realy play bg2+ToB with thief and when he reached some level he can choose ability to use all the item . And as i said before ,i played it myself so if there wernt a bug , than thief CAN use Holly av. to backstab.

BG2 - TOB spoilers...
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The ability to "Use Any Item" which is a thief HLA does not necessarily allow you to backstab with said item. Yes, it means that your thief can wield the Holy Avenger just the same as it allows you to read from scrolls or use wands. Yes, you can go invisible and whack someone from behind. But no, it does not apply the backstab multiplier to that attack, which kind of makes backstabbing pointless (other than to get a free attack on some unsuspecting sod).

The point SixOfSpades made is that the backstab multiplier will only be applied when a thief backstabs with a weapon normally useable by thieves. The Holy Avenger (which is coded to allow only paladins to use it), and all two-handed swords for that matter, can not be used normally by thieves, therefore the backstab multiplier is not used. The same thing is true for the Staff of the Magi (much to the chagrin of my F/M/T). Since it can only be used by a mage, it can not be used normally by thieves and thus does not permit the backstab multiplier to go into effect, even though other staves can be used for backstabbing...