Just know, devotion does not care about the graveyard, the stack, exile, or your hand
Doomsday Confluence X-cost spells, while potent in the late-game, can often be underwhelming inclusions at any other point of play
This one becomes a vehicular copy of a creature and demonstrates Magic's loosey-goosey willingness to use both spellings of 'impostor' and 'imposter.' Mech starts off as a noncreature with a crew cost that animates it later on
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The thing is while it is more likely to stay in play and get the job of keeping you alive than Sower it is probably just delaying the inevitable while if the Sower does stick she may turn the game round single handedly
With Ellie, you can use your life and a sacrifice to deal direct damage and provide protection