The Daikaisho is mentioned in the Chronicles as "a time when the forest suddenly boils over and rises like a tidal wave to cover the land. It is written that there have been three such waves since the Seven Days Of Fire." 
Daikaisho (大海嘯, daikaishou) means "Great Wave of the Sea". The last one occurred 300 years ago, destroying the Kingdom of Eftal. What brought about the destruction was a fight about the royal succession. Various factions armed themselves and arms merchants hunted the Ohmu for their shells. .
Finally, the Ohmu raged and overwhelmed Kingdom of Eftal for 20 days, destroying towns and cities. For 20 days, the Ohmu stampeded and destroyed, then they died, spreading the Sea of Corruption 2,000 leagues beyond where it started 20 days before.
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