Favor tokens to demonic runes is cool by me; I made up "favor tokens" because the original version had an unnamed resource, and it made a lot more concrete sense to name it in some way.
Ok, the ugly: I think the power level of the PP is more or less right -- but I do have an issue with how all over the map it is in terms of what it encourages in terms of your base character. I mean, you have:
F16: Necrotic damage, Abyss keyword (only exists in this PP, maybe in your related class)
U12: Melee Basic attacks.
Attack powers: Implement
So, what you're looking for is a class that mostly make necrotic melee basic attacks, but uses an implement.
That's what? Hexblade? Maybe Executioner Assassin with a Shadow Master Ki focus.
I'd probably:
Make the U12 affect all attacks. It's a daily -- a global damage buff is appropriate.
Make the F16 affect all necrotic attacks. At 16th level, you're not that far from getting the crit optiization available at epic levels; getting it for necrotic classes at 16 is fine.
That way, it ends up being a PP that boosts necrotic damage for classes that can use an implement and are (or can multiclass to) Arcane -- which is a perfectly reasonable set of requirements.