Firstly, I've gotta say I really am in love with light armor NBs and commend Zenimax for the overall skill design of the Shadow & Siphoning lines. I've always played Shadowpriests and Blood Mage styles in previous MMOs and simply love the healer/dark caster hybrid. Not the best damage or healing (but decent at both)- but strong debuffs, CC, DoTs, and resource management. I tell you this so you know my biased perspective. I'm not claiming to be objective, but simply pointing out a few of the skills that could use a little work as well as a minor dash of wish listing for "down the road" development.

Agony (& Morphs)
Everything is immune. This covers CC in general, so perhaps it's a "wait for broader changes" situation, but at the very least could it not be a strong DoT? Something along the lines of ~50% more than cripple. CC if possible, huge DoT if not. Balanced in that it has the cast time and up the magicka cost if needed. This skill sees no use and needs something.

Siphoning/Leeching Attacks
Love these skills. Tradeoffs like this are great for changing gameplay, but it could use a couple of tweaks for usability.

1) Toggle-skills in general. Can they PLEASE keep their toggled status on/off based on their previous state? The current implementation really forces you to play with it on both bars, or not use the skill at all. Eats too much time to be used in realistic situations.

2) Reducing weapon/spell power, rather than damage output. Maybe this is intended to benefit turtle builds that run without weapon damage/spell power (not that I personally have a problem with this) but it really limits build options when using this skill. Perhaps better if it instead just reduced overall damage output by ~8-10% or so. Gear agnostic. Again, unless the behavior is specifically intended, which I can respect.

Path of Darkness (& Morphs)
Okay, so I love the idea of this skill, but it just fails in action. It's a very magicka efficient skill, but simply has its power stretched out over too much time- oftentimes when NPCs die too quickly (It's just outclassed by Sap Essence here), or in PvP where players who just move around too much for it to be effective.. Plus, the movement speed boost feels misplaced...

1) Replace the move speed bonus with something else. Some ideas

-An Immovable-like bonus (just for the NB)?
-25% reduced damage taken from AoE for allies within?
-Enemies within take 15% more damage (from all sources?)

2) Condense the duration and damage down to 5-7 seconds. I think this one is an easy given, whether #1 is acceptable or not.

Dark Shades
Shadow Image has a unique teleport niche. The two shades are great at draining stamina in 1v1s, but feel lacking aside from this (likely not specifically intended?) side effect of the skill. Rather than the current tooltip of the shades doing reduced damage, it would do well for them to do increased damage. Hey, it might just be me, but comparing the finesse option of Shadow Image to the more brute force of Dark Shades, it simply doesn't come close- either in direct comparison or vs. other more useful skills. They just need to do more damage.

Santa, please bring me...
Okay, so long term (if new skills were added to existing skill lines) I feel Siphoning would be rounded out rather nicely by a health-cost skill. Yes, this absolutely piggybacks off health sacrifice skills from other games, but pairing life-steal style skills with health sacrifice skills has a proven track record of creating interesting gameplay. There's a certain ebb and flow to managing your health and magicka bars as offensive resources. That, and spamming Funnel Health feels both dull and contrary to the attached HoT benefit.

Call it Haunt/Echo/<insert appropriate name here>
(Possible skill for Soul Magic, rather than NB specific)
Instant cast, average magicka cost. Deals damage to the target (a decent amount more than Funnel Health) but you take the same amount of damage. 3 seconds later it deals a smaller amount of damage, and heals you for a small amount. This secondary effect's timer is reset by casting the skill again.

I like this design for two reasons- it gives a nice nuke with a health cost to mesh with with the rest of the Siphoning line. It also gives some reckless/responsible gameplay choices. Do you spam it, constantly damaging yourself but providing high DPS... or do you wait for a little extra damage and a heal to recoup a portion of the original health cost?

Alternate design: The secondary 3 second effect is removed. The skill starts only dealing 50% of the nuke damage to you, but increase with successive casts within 5 seconds of one another. 50% > 75% > 100% > 125% > 150%

Thanks for reading. I always appreciate comments/criticism!