Costs and conditions associated with attacking or blocking.

Some cards have effects that prevent specific creatures from attacking or blocking unless a cost of some kind is paid. For example, the controller of a cannot be attacked unless the attacking player pays two generic mana for each attacking creature. This cost is paid when attackers are declared, it is not something that uses the stack or can be responded to. Similarly, many creatures have abilities that prevent them from attacking or blocking if a specific condition is not met. For example,

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