The model is not Deployed as normal.
Instead, at the start of any Player Turn, it may choose a friendly Rank-and-File model, which satisfy the following criteria:
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The model with Hidden could normally have joined the unit.
The model with Hidden has an identical base size and Height as the Rank-and-File models in the unit.
The unit is not Shaken and is not a Summoned unit but it can be Engaged in Combat.
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After choosing an eligible unit, follow these steps:
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Remove a Rank-and-File model, with the same base size as the model with Hidden, from the front rank of the chosen unit as a casualty.
Deploy the model with Hidden where the removed model was positioned.
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If the chosen unit is Engaged in Combat when being joined by a model with Hidden, consider the model with Hidden to be in First Round of Combat for all rules purposes, with Duration: End of Turn. Note: Attachable models with Hidden can still be Deployed using the Hidden rules.