There's alot of discussion about this right now. I'm also getting alot of tells about this. so i'll make an attempt to describe it to you all.
Backstab is an additional attack gained from the Sneak Attack skill.
It can only be used from behind and while the target fights someone esle. This means you can only use it in a team.
Backstab is NOT a perk special, Do not mistake this for the Stab perk special you gain with Piercing Mastery 1, they are entirely diffrent.
Blackmane said i could quote his post from beta about Backstab.
Originally posted by Blackmane
Shades and adventurers receive a new special attack based off Sneak Attack called Backstab. A weapon that can perform a sneak attack special can perform a Backstab, *provided that you have earned the Backstab ability*.
The Backstab ability is not enabled by default on your character. It will become enabled when you have invested a certain number of points in your Sneak Attack skill. When you have done that, you will get a message that you can now perform a Backstab attack, and you will get a new icon in the GUI for this attack. The Backstab attack is not related in any manner to the perk special attacks.
Backstab locks the Sneak Attack skill. So you cannot use both a Backstab and Sneak Attack - you need to decide which one you want to use.
How does Backstab differ from Sneak Attack?
Backstab is not related to concealment in any way, so you can backstab a monster that is aware of your presence, you don't have to be concealed.
However, to perform a Backstab attack, you must be behind the monster and the monster must be fighting someone else. This means that you cannot perform a backstab if you are the only person fighting that monster.
Backstab also has 2 different power steps in its development. When you first enable the Backstab attack, it is very weak, because it doesn't yet have the increased critical chance of damage multipliers that are applied to a sneak attack. Once you have put enough IP into the sneak attack skill, you will first unlock the damage multiplier, and finally the critical increase.
So there are essentially 3 stages in the progression of the Backstab attack: enabled but weak, enabled and now with damage multipliers, fully enabled with damage multipliers and critical increase.
Remember - backstab becomes enabled and the second and third tiers enabled based on your unbuffed sneak attack skill, not your buffed skill.
The Shades are the masters of the Backstab, so while adventurers can also get Backstab, they must invest more points into sneak attack first of all to enable it, and secondly to reach the second and third tiers.
The skill levels are: (BASE Skill in Sneak attack).
Enabled: 100
Multipler: 235
Crit bonus: 420
You have to get above these levels for it to take affect.
You will also have to use a weapon capable of Sneak Attack.
As of today, 12th Sept -03. The Backstab icon is Bugged/has no icon.
If you use new GUI you will have to switch back to the old one, and open the action-tab. Find the noname icon and put it on your hotkey bar. Try it on some mob to find out if it's the right one. Should give an error message saying "Target must be fighting someone else". Once you have it on your hotkey bar. switch back to new GUI if you want to, and you're set to go.