Because of memory limitations, another part of the game (The Black Onyx) was released separately on some platforms as The Fire Crystal (ザ・ファイアクリスタル) (which added a magic system). Two other parts were announced, The Moonstone (ザ・ムーンストーン) (which allowed the party to explore the wilderness), and Arena (アリーナ) (which allowed the party to take part in Arena battles). Source: Wikipedia, "The Black Onyx", available under the CC-BY-SA License.