Moth Lords are FOEs in B3F of the Hall of Darkness. The fastest way to find them is to access the southwest room of the area, where 4 of them await. Like Big Moths, they are dormant until you enter a battle near them, upon which all of them will begin rushing the party. In-battle, the Moth's greatest attacks use arms - landing an arm bind will prevent most of its attacks from working so you don't have to worry about its Mince and Madness Spores.