Assassin.
(DF12),
(P#3:50 for Power-Ups)
Grab as Advantages: Night Vision, Striking ST (only on surprise attacks) and Luck.
As Power-Ups: Backstabbing (for when stealth fails), Silence, more Striking ST on surprise attacks, Weapon Master, Focused Fury if you more often need to down the target to start a fight, rather than down it silently and move forward, Bane-Brewer perk if you want to Bladeblack the most important targets.
The book recommends Trademark backstabbing moves. I
have created one that focus on looking cool, keeping the target silent and dealing quite some damage (it's not very optimized for damage, though), while staying strictly by the books.
However, that uses some
Martial Arts rules and your GM might want to restrict you on how much of the penalties you can buy off, sticking to
Slayer Training(
DF11).