her ability is an enter the battlefield trigger which is why torpor orb is in the deck because it stops her ability, you can also use endless whispers and give abyssal persecutor to an opponent, also you can put Phage's ability on the stack and then use sundial of the infinite to end the turn and it removes the ability that causes you to lose the game from the stack
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the reason its in there is to draw out the rest of the combo pieces and then once your set up you just use its ability from necrotic sliver to sac it and begin destroying all other players permanents