I have taken into consideration all of your posts, and Odin, I'm still debating whether or not to use your art, but I didn't alter the effect of draining HP because it really does take on the whole Darkness Vampire feel, but I did nuke the damage dealt quite a bit. I'm now contemplating making an 
Immortal killer card, because in legends, even immortals can be killed.
The vamp drain is okay. And, I definitely agree that it works well thematically and makes this card unique since there is no mass drain effect available yet. I was mainly concerned that the the damage was too high.
Dealing 8 is much better than 15. Its still very high, but at least some things will be able to survive after being buffed.
I think the unupped one should probably deal slightly lower damage (maybe 7 or 6?) in this case since creatures are generally weaker and there will be fewer buffs to help things survive in the unupgraded meta.
As for upgraded creatures. This is a tricky topic. Generally speaking, bypassing immaterial is something of a taboo. The status needs to be respected in terms of the value it adds to a card.
More specifically, all immaterial creatures have been balance wrt cost to account for their high level of protection. This additional cost needs to be respected when considering the balance of a card that can bypass the status.
For example: SoW bypasses immaterial status and the benefit to doing so is that it adds an additional effect.
Likewise if a negative effect bypasses immaterial status, there should be some kind of penalty for doing so.
You have some options there, although space is limited. Some examples:
-Do less damage to immaterial creatures (they tend to be low HP naturally)
-Remove or invert the drain for immaterial targets
-Give the owner of the immaterial target something in exchange...(e.g. return dead card to hand, give new cards, more hps ... etc.)
