to me it seems mortal blow just always misses. I know if it hit every time killing things would be to easy but i miss more tthen i hit . Are there any skills or items that would up my chance to hit with it?
You get a skill that increasces special landrate sometime after 3rd class change, not a dagger so dont know off hand what its called or what level ~.~
There are also C and B grade daggers with 'Mortal Strike'(formerly Might Mortal) SA, however they made it so you cant put the SA on anymore after mid-c2... so they are extreamly rare and pricey
And as the previous poster said, its not 100% because it does crazy amounts of damage. Daggers are based a lot on luck... If you get lucky you can kill someone in 1-2 shots...if you are REALLY REALLY unlucky you can spam your whole mana bar and not do any damage.