Characters can be unseen for a variety of reasons. Most commonly, they could be hidden, concealed by darkness, or by magic.
Unseen characters in other Zones cannot be attacked. Characters remove 2 dice when attacking an unseen character in the same zone. Characters add 1 die to all Attacks they make while unseen.
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If two characters can’t see each other, the bonus and penalty effects will both apply, resulting in a net 1 die penalty when attacking each other.