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Any suggestions to replace with Demonologist with what card? One problem I have is that not being able to draw Overseer or Vampiric Skull even when it's like mana 6 or 7 turn. Yes, sometimes my luck is that bad! Demonologist sort of solve that problem for me but yes, my early game will have to mainly rely on zombies (can be weak).

There's not really a replacement for it, you won't always draw your top end when you need it but giving up the tempo to draw it means you're less likely to be able to survive to play it. Having a stronger early game will let you live longer to naturally draw your top end.