There's already a feat that turns your necrotic powers into necrotic and poison, which makes them bypass simple necro resistance.
Actually that's possibly one of the worst possible choices you could make, unless you later take the feat to negate poison immunity. Most of the creatures resistant to necrotic damage - notably undead - are immune to poison. So all you're doing is turning your non-damaging effects into something most undead will now utterly ignore (while before you're just doing less damage). That's actually a perfect example of a "trap feat" in the game, because you're actually
worse with it against undead (who are widely poison immune, necrotic resistant).
So now you need an assassin multiclass (or be an assassin, in which case you probably weren't worried about necrotic much anyway), then venom hand master and have sunk 3 feats just to negate necrotic in an extremely roundabout manner. Additionally you're actually making life very hard for yourself because your non-damaging power properties are now ignored by undead (due to the poison keyword) - until you have the entire combination that is. The irony is substantial when you consider some of the HoS powers effects target undead, only to be negated because poison picks up all your non-damaging aspects of the power. The undead then become immune to these powers in the first place - which is pretty funny but won't please the player when the DM picks up on this. The necrotic at-will in the book, which cannot negate an undead creatures regeneration or give it vulnerable because you added the poison keyword is pretty funny.
But you can do it this way, it's just grossly ineffective and unless you plan to MC Assassin, practically undesirable. You can actually make all of your powers
worse this way, attempting to circumvent the resistance of the same enemies that are causing you the most problems in the first place.
Edit: This requires a direct rules citation.
Page 225 of the Rules Compendium said:
If a creature is immune to charm, fear, illusion, or poison, it is unaffected by the non-damaging effects of a power that has that keyword
In fairness, this is a remarkably overlooked and overly complex rule that
shouldn't be there. But poison is literally and do forgive me here, the
poisoned chalice of damage types in 4E. You NEVER add it to a power unless you can get away with it because of this.
Now, One With Shadow was a minor action in the early draft, and at some point during R&D it was (probably correctly) that this was just too powerful, and it ended up a standard action. I think that might be too harsh of a penalty (personally, I'd keep it a Standard and include the actual rolling for Stealth into the same action).
I think a move action would have been okay myself and been tactically useful. It's a bit easier to generate more move actions - or get a move before an attack (as an example, Deft Strike) than a standard. The standard action practically cripples its combat use and that the feature actually isn't worthwhile outside of combat has been discussed pretty extensively.
In the original design, the vryloka (still named the vrykolaka) had a racial power triggered by killing a nonminion enemy, that allowed them to spend their second wind and regain additional hp equal to their surge value. That power would be most helpful when the vryloka was bloodied, but then their surge value would be reduced by 2. So the player had to decide "do I use it when I can get the most hp out of it, or when I need it the most?", which I thought was interesting.
That is actually pretty interesting and I think would have made the penalty fit a bit better in context. I still don't really see any point for the -2 penalty, but at least it doesn't really bother the Vryloka as a race that much after a few levels. Ironically I read elsewhere someone complained that the penalty should
scale so that it wasn't a huge disadvantage early, then irrelevant later on!