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Achieving balance with 30 cards is very difficult indeed. But as almost all mono decks need to emphasize speed, all modifications i did were mostly to improve speed.
-2 Dragons to make it faster, +1 Tower for better % with starting hand as this deck will always be faster with at least 2 towers in the opening hand.
+2 Nightfall. For faster damage, faster healing and generally faster games. Also it makes sure you always have one in case it gets destroyed. IMO an early Nightfall is tons better than an early dragon as it greatly improves all your creature's survivability.
-1 Siphon Life. This spell will almost always be played only when you're about to win, about to die or for killing your natural enemies, Otyugh and creatures with Butterfly Effect.
-1 Improved Steal. With the amount of healing you get from Eclipsed Vampires, you will rarely need to steal anything. You will want to reserve this for pulverizer, electrocutor, fire shield and spike carapace.
+1 Vampire Dagger. This is arguable, but it grants damage/healing so this will almost always be staple to a mono darkness deck. I do replace this sometimes with another Improved Steal, Liquid Vampire or Obsidian Dragon.
I actually use this deck in PvP2 and farming Top 50. It's very fast and efficient. But for better win % i add 2 Fractals and Aether Mark. All other cards are the same.