This illustration looks to be inspired by two different official FF6 illustrations by Amano Yoshitaka: the airship and spired city in the background, along with Terra's upright posture seem to come from post #1013533, while the details of her Magitek Armor (particularly the legs) and its orientation seems derived from post #309846. Could be considered derivative work, but I think there's enough reinterpretation that it's not needed.