[ Disclaimer, --- , ]
This rogue only skill can inflict enormous damage with a piercing weapon. The target must have its back to you - otherwise, the backstab will just be a regular attack. Higher skill will increase the damage done.
The damage done is multiplied by your backstabbing weapon's damage. The higher the damage the higher the backstab.
Because of this, it is always a good idea for a Rogue to have the highest possible damage dagger in their primary.
Allegedly has a cooldown of around 10 seconds.
See Game Mechanics article for the Math behind Backstab damage.