Kumano Taisha was the most important shrine in Izumo for centuries before Izumo Taisha supplanted it in the late Heian Period. Empress Saimei ordered the shrine built in the mid 7th Century. Before that the shrine was the mountain behind it.
The kami is Susano, which is not surprising since it is located in the heart of Susano country. There are numerous other shrines within the compund, the two major ones being to Izanami, Susano's "mother", and Kushinada, his "wife".
No one can know for sure why Saimei ordered its construction, but it seems likely to me that she was engaged in attempt to unify the provinces under Yamato control to fight against the threat of Sila. She died leading an army in Kyushu on the way to the Korean Peninsula to help their ally (relatives?) in Paekche who was threatened by Sila. After her death the Yamato forces suffered a humiliating defeat by Sila so it is not often mentioned in histories.
An unusual building with a thatched rood and walls covered in cedar bark houses sacred fire-making tools used in rituals at Izumo Taisha, the fire made from the tools is used to cook rice to be offered to the gods. Before Meiji the fire would also have been used to cook the food for the new head priest rituals.
The annual Sankasai ritual is when the priests come from Izumo Taisha to borrow the fire-making tools. They bring with them a long rice cake as an offering. The Kumano priests complain about the rice cake and insist it should be made according to their specifications, but eventually relent and make it themselves as it is too far for the Izumo taisha priests to go back to make a better cake, but the Kumano priests insist that next year the Izumo priests must make a better cake.