is it the dark mater?my logic on this is way wrong an half asleep so not readin ya hints well enuf but dark mater is the second to last materia to be used at the end, as Holy is the last, and as to the second found, in the ancients temple u find bahamut summon first, if i remember right (might b wrong as not played in years an my PS n players guide is in attic as room is empty at min)
