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These offer a few cards from a lot of series to choose from

Replace it with something that reduces mana cost rather than eliminates it entirely, like As Foretold or Baral Chief of Compliance

Many of these creatures have powerful secondary abilities including creating additional creature tokens (Bladewing, Death Tyrant, Feldon, Kari Zev, Krenko, Mardu Strike leader), drain/life loss (Calculating Lich, Hellrider), direct damage (Combustible Gearhulk, Dreadhorde Butcher), anthem/buff effects for your army (Drana), card draw/free spells (Etali, Florian, Garna), additional combat step (Moraug), and many more

With this logic in mind, this is how you are going to start picking your cards