it would be best to play endless whispers, and then immediately sacrifice phage in the same main phase.
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the only problem here is if you play against a black deck, half of your kill cards are "nonblack creature" making yourself sacrifice your own creature (phage) would be much more efficient. if you have a bunch of their creatures and they use a wrath of god or card that clears the board equally, now they have 10 creatures and you have none