Spoiler Alert! We have some more information about how to perform this quest but recognize you might want to try it on your own before we tell you the secret...
OMG! The "New Moon" Beast is a very large Gargoyle. Before you could even engage it, Sismond appeared and destroyed it. It is kinda implied that he IS the "mysterious stranger" of legend that fought this beast before. Although he could not destroy it back then he must have left an alarm in case it was ever awakened. Now he has the power to end it permanently. He warned you not to interfere with him, or he would have to kill you. Apparently he does not really want to. Anyway, he quickly vanished.