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After you have Terra back in your party (after the Esper deal), go to Narshe, to the house in the right hand corner. You'll find Lone Wolf there, and he'll run off. Follow him up the mountain, and he'll have Mog hostage. Just sit still and Mog will get up, and both will be thrown near the cliff's edge. You have the choice of saving Mog or Lone Wolf, save Mog. You have to complete this before the World of Ruin.