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If Mirrorhall Mimic somehow enters the battlefield at the same time as another creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.
If another creature becomes a copy of Mirrorhall Mimic, that creature is also a Spirit.
Any enters-the-battlefield abilities of the copied creature will trigger when Mirrorhall Mimic enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.
If the chosen creature is a token, Mirrorhall Mimic copies the original characteristics of that token as stated by the effect that put the token onto the battlefield, except it's a Spirit. Mirrorhall Mimic doesn't become a token in this case.
If the chosen creature is copying something else (for example, if the chosen creature is another Mirrorhall Mimic), then Mirrorhall Mimic enters the battlefield as whatever the chosen creature copied.
Mirrorhall Mimic copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below), except that it's also a Spirit. It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on. Most notably, if it copies a creature that's not normally a creature, it won't be a creature.
If the copied creature has in its mana cost, X is considered to be 0.
