The Royal Sword, sometimes translated as the EmperorSD or EmpireSD is a recurring weapon throughout the Breath of Fire series. It is usually one of the statistically strongest weapons for Ryu, usually does lightning elemental damage, and often requires the player to search the world for its secret location.
Breath of Fire
Translated as the EmperorSD, it is the most powerful weapon for Ryu with an ATK Power of 255 and a Weight of 1, the weapon can be used as an item to cast the spell Char. It is an easily missable item that can be obtained in Emperor Jade's throne room and is hidden on the left side of the throne. It can be acquired only after defeating Jade. Due to to being obtained so late in the game, the weapon is not as useful as it seems as the final boss Tyr is the only enemy left to fight.
Breath of Fire II
Translated as the EmpireSD, it is the strongest weapon for Ryu Bateson with an ATK Power of 160, an innate element of Electric, and can be used as an item to cast S.Boom. The weapon is easily missable, appearing in a second fishing spot near Township that is unreachable by walking. The player needs the power of the Great Bird to reach the spot and will need the Charm Rod to fish it out from the lone treasure chest in the fishing spot. It is a missable item as the player needs to obtain it before the party reaches Gate and loses the ability to use the Great Bird. Township is too big to land on the stretch of land where the fishing spot is.
Breath of Fire III
Known as the Royal Sword, Emperor Sword (こうていの剣, Koutei no Tsurugi?), it is statistically the second strongest weapon for Ryu with the Goo King Sword being stronger. The weapon has an ATK Power of 158, a Weight of 8, and has an innate Electric element. It can be used as an item in battle to cast Lightning.
Unlike previous versions installments, the Royal Sword can be obtained two different ways. One way is an optional side quest through the Desert of Death which is actually referenced in-game by an old man in the Oasis who mentions dropping an "important item" while traveling through the desert. The player then needs to re-enter the Desert of Death and turn slightly from the west and travel for three nights. After the third night, turn directly west and keep walking until you find the weapon, it should occur in the course of one night so if the player walks longer than that, they missed it and need to try again.
The second way to obtain the sword is by trading for it in the Manillo Shop in the Lost Shore Region east of Kombinat. Use a Coin bait to lure the Manillo and simply trade 1 Whale, 3 Spearfish, and 1 Barandy to obtain the sword.
Breath of Fire IV
The Royal Sword is the default weapon for Fou-Lu, and is the best multi-hit weapon for Ryu. The sword has an ATK Power of 96, Hits 2x, and has a Weight of 6. It can be used as an item in battle to cast Drain. Fou-Lu begins the game with the weapon and it is his strongest weapon throughout his chapters. The player can obtain the weapon for Ryu by stealing it from Astral.