Moeru Ryusoken: Kaen Ryou— Fire Flame Dragon King—is among the many powerful techniques that Sabo has under his name in the One Piece series. It is an attack where Sabo fuses his mastery of the Dragon Claw Fist, Ryusoken, with his new powers from the Flame-Flame Fruit to create something truly devastating in terms of martial arts and fire manipulation fusion.
The technique's name in Japanese is "Moeru Ryūsoken: Kaen Ryō", which translates to "Burning Dragon Claw Fist: Fire Flame Dragon King". In official English translations, it has been referred to as "Burning Dragon Claw Fist: Flame Dragon King" in the Viz Manga and Funimation dub, and "Blazing Dragon King" in other translations. At a glance, these variations in naming illustrate how tricky Japanese attack names can be to convey a feel of within translation.
This was a technique developed by Sabo to merge his already-enshrined skills in martial arts with the powers inherited from Ace's Flame-Flame Fruit. The Dragon Claw Fist style is the foundation for this attack, wherein Sabo acquired it from the Revolutionary Army. He blended this martial skill with the intense heat and destructive power from the Flame Flame Fruit to come up with a very distinctive, extremely formidable fighting style.
One of the more striking visuals would be Fire Flame Dragon King, an anime representation. Typically, every time Sabo executes this technique, his fist will be covered in really furious flames that resemble a dragon's head. This is not only a visual effect showing just how strong the attack is but also represents the underlying dragon theme with Ryusoken.
Fire Flame Dragon King shows how much Sabo has matured as a warrior and his ability for flexibility, evolution, and innovation in battle. Coupling the Ryusoken—a real martial art he had perfected over years—with the more recent abilities of the Flame-Flame Fruit, Sabo reveals just how creative and tactically effective he can be in combat. This union of skills makes one a formidable opponent, for he can surprise seasoned veterans with just that bit of mix between the expected martial prowess and elemental energy.
Ever since Sabo ingested the Flame-Flame Fruit, the technique has been in major fights. In the Dressrosa Arc, for example, Sabo used this attack against several powerful enemies pretty effectively. It also became very destructive, as observed with the Fire Flame Dragon King, where Sabo was able to fight with high-level foes, showcasing its strength from the combined ability.
Fire Flame Dragon King stands for a different approach to power within the greater scheme of things across the One Piece world. Most characters are focused on Devil Fruit abilities or martial arts. Sabo's technique showed that these two systems of power could complement each other. In this sense, this approach toward combat goes very well with the concept of progressive growth and adaptation of the series, since characters find new ways to gain strength while opponents they have to fight gradually get stronger.
Because of its popularity among fans, this has seen Fire Flame Dragon King being applied in a number of One Piece media other than in the manga and anime. An example is in the One Piece Trading Card Game, where there is a card called "Flame Dragon King" that allows players to raise the power of their Leader or Character cards during a fight. This is an addition into the game of the relevance of the technique and has proved quite attractive to fans of this series.
It is essential to note that the development of Fire Flame Dragon King serves as a touching tribute to Ace, who was Sabo's sworn brother. By incorporating Flame-Flame Fruit abilities into his fighting style, Sabo does not only boost his combat powers but carries on the legacy Ace has left behind. This depth in emotion adds further meaning to the technique and makes it not just a very strong attack but a gesture of sworn brotherhood between the two brothers.