Blade of Woe (as long as your targets are NPCs)

Join the Dark Brotherhood. That guild questline grants a skill (not a weapon skill, but a special power) that one-shots non-elite enemy NPCs. (It also produces cool cinematic kills. It was clearly meant to give something like the Skyrim dagger-assassin experience.)

If you don't have that DLC, you have two options.

- An Onslaught build. The Two-Handed ultimate, Onslaught, can be made sufficiently powerful to oneshot most enemy NPCs, and it refunds its entire ultimate cost on kill, so you can use it again on the next baddie. To maximize Onslaught damage and ensure oneshots with it, you probably want to wear the Sword-Singer set in combination with another weapon/physical damage set (Automaton, Clever Alchemist, even Hunding's). Con: not daggers. As you mentioned, sneakily killing someone with huge two-hander is not very immersive.
- A dual wield heavy attack build. This works best on Dragonknights, who get insane heavy-attack damage, but it can probably work on a Nightblade, too, since enemy overland NPCs are so weak. You probably want to wear Automaton, Clever Alchemist, and one piece of Kena or Velidreth, to maximize physical damage. (There are sets in the game that maximize heavy attacks and sets that maximize damage from stealth, but unfortunately they're for bow users and magicka users.) Then sneak up to the baddies and heavy attack them. Cons: gathering those sets is a lot of work, and again, it's not very immersive, since your assassination skills are coming entirely from gear. Also, heavy attacking doesn't feel very assassiny.

If this is what you're looking for, Blade of Woe is what you want (Only works on NPC's)