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Maybe they're bugged. I haven't yet tried out a Nightblade, so try /bug report and explain what you are seeing. Only other "rouge" type characters I've played in other MMOs tend to have either positional or weapon specific attacks (or both). Re-read the skills explanation it might provide a clue.
My Shadow skill looks bugged too. It won't level past 2 and I need to get it to 4 at least so I can get Veiled Strike and higher so I can morph that to Concealed Weapon for my build.
What is the status on this and its a shame to discover this now at level 11 already?
[EDIT:] Well I currently have to redact just about everything. After completing a dungeon sneaking around and reaching level 11 I discovered Veiled Strike available. I also completed the first part of Long Lost Lore solo and learned that the constraint that a couple dungeons I thought were 'solo only' doesn't really exist. Sure you can play these dungeons solo but you don't have to if you've got a group to make matters 'easy'.
[EDIT:] There remains a problem here. It is clear as mud whether a dungeon is solo only or a group (formed on the fly) can enter. Zenimax should clarify this via map symbol and map key imo.