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The dagger symbol which the Moiety use is based on the shape of one of the giant daggers which Catherine Wrote into the Age of Riven during her attempt to escape from Gehn with Atrus. Several Rivenese people observed the events leading to Gehn's imprisonment on their Age. From this observation, they came to believe that Catherine was to be their savior, having joined Atrus, whom they deemed to be the true god, sent to punish Gehn for his blasphemy.
Thus, the dagger beside the Star Fissure became a religious symbol to them, and they left it as a sign of their presence whenever they reappeared on the islands of Riven. The Moiety never used the dagger as a weapon, however. It was purely symbolic of their cause and their presence. In conditions where there is little light and Daggers need to be placed, the Moiety simply place firemarbles on the vertical bar inside the circular part of one.