Rordan's Backstab doesn't appear to do anything. I'm not sure why, or if there are any conditions for that to be the case. There aren't any conditions listed on the tooltip -- it just says it deals medium damage that is unguardable. But it's not dealing any damage at all, nor is there any other effect.
It was working perfectly fine on a previous version, but if I remember correctly, it also had an additional effect (I seem to recall it also making a Bleed gem before?) Maybe when that was removed, the damage was accidentally removed as well.