Rengoku (Purgatory) was an ironclad battleship purchased and commanded by Shishio Makoto as the intended final stage of his attempted rebellion against the Meiji Government in 1876.

Purchased from the branch of the Chinese Mafia headed by Yukishiro Enishi, the Rengoku is said to have cost fully three-fifths of Shishio's fortune to acquire and prepare. Even so, as an example of one of the nineteenth century's "three great machines of war," the fact that a single Japanese citizen was capable of owning one privately was--in itself--a sign of the Meiji Government's security failings. In addition to its inherent danger as one of the feared "black ships", the Rengoku's destructive capabilities were severely augmented by the presence of the other two great war machines as part of its artillery. The ship carried a Gatling gun on deck and was outfitted with at least one--and possibly as many as three--Armstrong cannons.

Set to sail covertly from the shores of Kyoto in the summer of 1876, the purpose of the Rengoku was to arrive in Tokyo's Yokohama harbor and begin firing on Japan's capitol city in the dead of night, inciting mass panic among the citizens while the members of Shishio's personal army--the Juppongatana--assassinated key government officials in the ensuing chaos. As such, the Rengoku was outfitted with a false shell made to look like an aged, harmless fishing vessel until the time came for its attack on Tokyo to begin.

However, upon the arrival of Himura Kenshin and his allies, Saitō Hajime and Sagara Sanosuke at the Kyoto harbor moments before the start of the operation, Shishio decided to shed the battleship's disguise early as a show of power to his three would-be foes. After firing the Armstrong cannon and Gatling gun at the approaching enemies to no avail, the Rengoku was unexpectedly struck by a trio of experimental grenades thrown by Sagara. Surprisingly, the small grenades exploded with tremendous force, breaking through the ship's steel armor, severely damaging the engine room and completely destroying the screw shaft. As a result, the Rengoku was fatally crippled before it could sail away from Kyoto and was promptly abandoned by its surviving crew.

Later on, Enishi, enraged at Kenshin for saying the name of his sister Yukishiro Tomoe, threatens to send 10 Rengoku ships to firebomb Tokyo if he ever does so again.

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