The name of the Nikko derived from Mt. Futara (Today's Mt. Nantai).
Mt. Futara also is pronounced as Mt. Nikou.
There are three possible reasons why this place
was named as Nikko.
The name of the Futara came from Potakala in Sanskrit. The Potakala
means pure land of Kannon Bosatsu. That word became Hodaraku in
Japanese, and became Futara then. Also, there is another possible
reason. Kuma-zasa (roughly mean bamboo leaves) grow gregariously
in Nikko, but the Kuma-zasa is pronounced Futora in Ainu. As you
imagine, the Futora became Futara. In addition to that, two storms
come to Nikko in the spring and fall. The word "two" sometimes
is pronounced "futa" in Japanese, and the storm is pronounced
"arashi" or can be shortened to "ara" in Japanese.
In this sense, the "two storm" became Futara.
There is a famous phrase to express Nikko. "Never
say great without seeing Nikko." There are many sight seeing
spots in Niiko as quoated. Generally speaking, Toshogu Shrine is
the most famous, but history of Nikko goes back to 766 in Nara period,
when Priest Shoto built Shihonryuji Temple.
Nikko is not only focused on relics, but also is
focused on plentiful nature such as rich forest and clean water.
Why don't you try to look at Mt. Nantai wherever you visit relics?
Appearance of Mt. Nantai must be changed by any of your location.