The drawback of not being able to block with your tokens is negligible since you're Golgari, and you don't let good creatures go to waste
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In that case, Malleable Impostor simply enters the battlefield as a 0/0 creature and is probably put into your graveyard immediately, unless something else is increasing its toughness to keep it alive
The single best card for Karrthus is Death by Dragons
Gix, Yawgmoth Praetor Life payments and card draw often go hand-in-hand in black, so Gix, Yawgmoth Praetor fits perfectly