MTG Unfinity released on October 7, and already multiple images have appeared on social media of cards with the wrong stamp
But at that point, you should stop and consider if you really need to add more Ink each turn
If the target creature is an illegal target when Fell the Mighty tries to resolve, Fell the Mighty won't resolve and none of its effects will happen
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Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke
Most of the time it plays like a Time Walk with a full Blatant Thievery attached to it