Lol this thread is getting interesting...
back to the card:
I like the idea of such legendary cards I too have tried to balance ones like this and they tend to get into stickiness. So to discuss that:
1. The Quanta gain: Interesting idea to use the Attack to take Quanta. Personnally I think that may be a bit much as with Blessings, Rage Potion (coupled with Basilisk Blood) etc you could easily get something that is way too good. In saying this I think the Quanta Gain should stay it is integral to the way you have put the card forward.
Suggestions:
a) Quanta Gain is Attack / 2 Rounded down (as suggested)
b) Quanta Gain is limited to 5 Quanta (Base Stats only)
c) Quanta Gain works differently: On each successful Attack Drain X Quanta from your opponent and you gain X/2
(whhere X is your attack) This way you still drain the same amount but you only get back half.
2. The Healing (vamp ability): Great to have this on another creature as a standard. Personally not much to change here as the Healing isn't much more than a Vampire even with Eclipse it is only 8 per turn and you can only have one of these in play therefore not over the top.
3. Another idea is that it comes into play Immortal thus you can;t boost it in anyway. Then it stays at 5 Quanta Drain and 5 Healing.
SO in short yes there are things that can be changed I hope these suggestions help somewhat.
As to raising cost to try and balance. Yes you are right that that doesn;t necessarily work but it can help to slow down the playing of the card. The issue to take into account is what will the card do once in play. To achieve this maybe a scarifice is called for before playing the card (ie Scarifice Target Creature with Equal or More Hp and Play Bloodshadow for 12
) I am unsure if that would work either but it's something to think about.
Thanks for listening to my ramble.
Scyther