- Name's the Same: The "ken" in Hadouken can refer to both "fist" as well as "sword."
- No Export for You: The original game never left Japan, so the only exposure in the West was just at Castlevania: Harmony of Despair (and Fuma himself only made cameos in Japanese games back then). Undying Moon averts this and also comes with an emulation of the original game, allowing players to experience the original game, but it was left untranslated.
- Sequel Gap: Getsu Fuma Den: Undying Moon will release on 2022, a full 35 years since the original game.
- What Could Have Been: A proposed sequel, Shin Getsu Fuma Den, has been trademarked as a PlayStation 2 title, but the game was shelved even before it was made. Undying Moon has no relations with it.
Trivia / Getsu Fuma Den