In an Empire as old and traditional as Rokugan, the Battle of the Cresting Wave is recent history. Three hundred years ago, a great and powerful oni lord called The Maw marshalled the forces of the Shadowlands Hordes and struck for the Empire like lightning. The Hiruma provinces, the closest Crab holdings to the Shadowlands, were overrun without warning. Many Hiruma died and the rest were routed, forced to fall back to Hida lands. There, the surviving Hiruma delivered grave news: the dark army that sacked Castle Hiruma was greater than any witnessed since the war with Fu Leng. With that forewarning, the Hida called upon the full might of the Crab Clan to withstand the advancing army.
However, one does not bring to bear an entire Clan's military overnight. Fortifications, musters, reconnaissance, strategy, and deployment take time. And days after the Hiruma survivors arrived, word from the invaded Kuni lands revealed the breadth of the Maw's forces. And the Maw himself marched for the Seigo River. Beyond the Seigo lied Kyuden Hida (Castle Hida) and beyond that, the unprepared and unprotected Empire. Hida Banuken, the Crab Clan champion, decided that the Crab would stand or fall on the north bank of Seigo River.
A battle in the open field would be disastrous for the Crab. Banuken had 20,000 men available from local forces, ronin, and nearby detachments of the Crane Clan; the Maws' army vastly outnumbered them. The Kaiu could erect fortifications in the field but that would take time and the Crab had about 2 days. A Kuni shugenja named Kuni Ozaku stepped forward. She asked Hida Banuken how long the Kaiu would need. The Kaiu said proper fortifications would take months to erect. Kuni Ozaku promised it to them and departed immediately for the north bank of the Seigo. There, Kuni Ozaku cast a powerful version of the Cresting Wave spell. The waters of the Seigo formed a wall stretching to the sky and as far as one could see in either direction. The Maw sent detachments forward to break through the wave but they were a handful of straw cast against a stone. Many among the Crab thought Ozaku a fool. Such a powerful spell would drain her in hours, not months. Yet hour after hour, day after day, week after week, Ozaku's spell held the Maw and his forces at bay. On the 73rd day, Kuni Ozaku breathed her last. Her Cresting Wave collapsed and the Maw was waiting. However, he no longer faced 20,000 soldiers in an open field. The Kaiu had erected their wall in half the time and they had built tunnels stretching beneath the Maw.
The Maw rallied his forces to crash against the fortifications again and again. And time and again, the Hida, Kaiu, and Kuni turned them back with blade, steel, and magic. All through the day the walls held and the continued clashes drove the Hida and the Maw to exhaustion. Thanks to the Kaiu walls, Hida Banuken did not have to commit his reserves during the battle. Now, at the end of the day, Banuken ordered his 10,000 Hiruma reserves, frothing with rage at the loss of their homes, through the Kaiu tunnels beneath the Maw. They emerged behind the Maw's forces and unleashed their fury upon the unsuspecting Horde. The Maw himself fell to Hiruma steel and Kuni magic, vengeance for their lost lands. The Maw's skull was hung from the walls of Castle Hida, in full view of the Shadowladnds, where it remains to this day as a warning to the Horde that even its greatest champion has fallen to the Crab.
The nascent fortifications were repaired, enlarged, and extended. This is now the Kaiu Wall. The Seigo River was renamed the Saigo no Kamae, or the River of the Last Stand. The Crab had to cede the conquered Hiruma and Kuni lands but the Kaiu Wall now stands as a line in the sand.