Official Rulings
In my battleline I have my opponent’s Dark Harbinger (MM 381). On my turn, I play Mimicry (COTA 328) to copy a Shadows card from my opponent's discard pile. Does Dark Harbinger's ability resolve?
No. Mimicry was played as a copy of the Shadows action in your discard pile, so Dark Harbinger will not resolve.
Source: Rulebook
In my battleline I have my opponent’s Dark Harbinger (MM 381). On my turn, I play Mimicry (COTA 328) to copy a Shadows card from my opponent's discard pile. Does Dark Harbinger's ability resolve?
No. Mimicry was played as a copy of the Shadows action in your discard pile, so Dark Harbinger will not resolve.
Source: Rulebook
Related Cards
Dark Harbinger always appears in a deck with at least 1 of the Mutations:
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