|80/100 giving extra checks?||yes|
|Obtainable in game?||yes|
Influences chance of extra damage on monsters, who have not noticed PC or are non-hostile.
This skill increases your chance to hit and your chance to score a critical hit when you attack friendly or neutral monsters (or on hostile monsters if they don't notice you). Note that daggers and knives are particularly effective when backstabbing others.
80 and 100 give extra chances to get backstab bonus which is not guaranteed [and rather rare at low skill levels]. Damage boost remains unchanged, at 2x. This bonus stacks with critical hits for total 4x.
Keep in mind that is easier to sneak up on monsters for backstab attacks if invisible or in darkness. Darkness also is the only case where the PC can keep backstabbing the same target continuously (even the PC attacked it before casting darkness).
Backstabbing seems to the give normal amount of weapon marks.
If the PC is a Thief or Assassin wielding a dagger or knife, they get an extra damage bonus when backstabbing; this increases the damage multiplier to 5x for Thieves and Assassins below level 12, and to 7x with the level 12 backstabbing ability. Other classes do not seem to get additional benefits to backstabbing from wielding a dagger or knife, despite what it says in the manual.
Bonus towards to hit still needs testing.
Successfully backstabbing enemies (which results in the message "You stab the -foo-" or "You splatter the -foo-", presumably depending on the type of weapon used).