Heigen no Kaminari

Shrine
The Plain of Thunder is the location of a shrine dedicated to Osano-Wo, the Fortune of Fire and Thunder.

Emerald Champion
The plain is also the where the Test of the Emerald Champion is held, in honor of the fortune who is the patron of all bushi.

Location
The plain is located between Unicorn and Scorpion lands.

Major References

 * Legend of the Five Rings: Third Edition Pages 304