If the copied creature had in its mana cost, X is 0
This actually led to The Immortal Throne receiving an official FAQ nerf
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Consequently, Inquisition of Kozilek basically ends up functioning as Thoughtseize, without costing you the additional life loss
Other stores opt to have a slightly more beginner-friendly model for their prereleases and provide participants with a starter deck and a select number of packs from the new set
In an aggressive deck, this is likely to be more than enough to kill them off - and that's not accounting for any extra creatures you may have, any ramping you've done or spare mana you've created
