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
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The single best card for Karrthus is Death by Dragons
If youre a more casual Magic financier and you either have plenty of capital or an aversion to quick-flip speculation, keep doing what youre doing
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