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If my oppenent casts Slum Reaper and I counter with Essence Scatter, does he still gain +1 life since he has Staff of the Death Magus in play?
Staff of the Death Magus triggers when the spell is cast. It doesn't care whether the spell actually resolves and countering the spell won't counter the triggered ability.
Yes. The stack doesn't matter when dealing with any of the Staffs. The card text reads Whenever you CAST a color spell....
So the effect is instant once you place that card on the stack.
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