Lurker is a Loaded Keyword that modifies the sequence in which skirmishes must be resolved. The Lurker Keyword had its premiere in the Shadows Expansion.
From the Comprehensive Rules 4.0:
During the skirmish phase(s), the Free Peoples player must resolve all skirmishes involving minions who do not have the lurker keyword before he may choose to resolve any skirmishes involving one or more minions who do have it.