This is the effect of SL Guardian Ring
[soul Linker of The Guardian]Str +20, Vit +40, Max HP +40%, Max SP +25%, Atk +15%, Hit +50, Walking speed +15%Reduce Vit def by 10%, 6% chance to cast Back Stab with increased damage of 60% when doing normal attacks.Reduce damage taken from Demihumans by 5%
Someone tested this on me, and obviously, back stab only works when the opponent is at his back. It takes time to do this and you can't really use the auto-cast skill properly as the opponent always has the chance to face you.
Not only that, it sucks since the damage is more or less atleast 40-70k each backstab hit and it makes your opponent face you after doing so, disabling you from repeating this skill again.
My suggestion is to change the auto-cast skill to any of these:
Magnum Break with increased damage of ??% when doing normal attacks (My top option)
Raging Quadruple Blow with increased damge of ??% when doing normal attacks (My 2nd option)
Meteor Assault with increased damage of ??% when doing normal attacks
Tarot card of Fate when doing normal attacks
Envenom with increased damage of ??% when doing normal attacks