When Osano-Wo was killed, his Matsu-born son Kaimetsu-Uo hunted down the killer and exacted revenge. His Hida-born son Hida Kenzan was not able to join Kaimetsu-Uo because the duties of Crab Clan Champion fell to him and the Clan needed guidance in the wake of Osano-Wo's death.
After he ascended to as Fortune of Thunder & Fire, Osano-Wo gifted his son Kenzan with the Tetsubo of Thunder & Fire. It is a large weapon, even by Crab standards, studded with eternal jade (it will protect against the Shadowlands taint without corrupting) and carved with the image of the Thunder Dragon on all sides. Kenzan left the weapon in a shrine dedicated to his father on an island off the Crab coast. Centuries later it was discovered by Moshi Iongi, a Centipede shugenja who would later join the Bat Clan in the Islands of Silk and Spice. Yorimoto Komori, the former Mantis and founder of the Bat Clan, would proclaim it the ancestral weapon of the Bat Clan.
In Legend of the Five Rings, the weapon grants a +2k0 bonus to all attacks and a +0k1 bonus to damage against Shadowlands creatures. However, for each 1 the wielder rolls on an attack or damage roll with the weapon, he takes 1 wound of damage for failing to live up to Osano-Wo's expectations (the Fortune is very exacting).
In 4e, I imagine a brutal weapon that lets you reroll attack rolls on a 1 (but you lose a healing surge to do it).