Fu-Rin-Ka-Zan is a common name for the phrase of 'as fast as the wind, as quiet as the forest, as daring as fire, and as immovable as the mountain,' written on the hatasashi-mono (battle flags) of Shingen TAKEDA, a daimyo (Japanese territorial lord) in the Sengoku period in the Kai Province (Yamanashi Prefecture). The flag was actually used. It is often used in war chronicles after the Edo period as something to remind of the Takeda army.
The phrase was quoted from a part written about the attack/withdrawal of an army in the battle section in "Sonshi" (Chinese books about tactics) which says ''as fast as the wind, as quiet as the forest, as daring as fire, as hard to know as dark, as immovable as the mountain, and the movement is like pealing thunder.'
This means, 'when you move, you do so fast like wind; when you stop, you do so quietly like forest; when you attack, you do so like fire; when you hide, you do so like darkness; when you defend, you do so like a mountain; when you appear, you do so suddenly like a thunder.'
Sometimes 'as hard to know as dark' and 'as immovable as the mountain' are written in a different order depending on the version of the book, and it is thought that 'Fu-Rin-Ka-Zan' were taken from the first four phrases in one of those books.
Shingen TAKEDA was not the first to use the battle flag of Fu-Rin-Ka-Zan. In fact, 200 years before Shingen, Akiie KITABATAKE, who was a young court noble and busho (Japanese military commander) as well as Chinju-fu shogun (Commander-in-Chief of the Defense of the North) in the period of the Northern and Southern Courts (Japan), used the emblem of Fu-Rin-Ka-Zan on a flag when he raised an army at Mutsu Taga-jo Province (present Tagajo City, Miyagi Prefecture) in order to defeat Takauji ASHIKAGA, who took control of Kyoto.
Akiie KITABATAKE's ancestor was the Murakami-Genji clan, who were active from the late Heian period through the late Kamakura period. It is thought that Akiie was deeply erudite, and was wholly committed to the thoughts of Sonshi. Akiie KITABATAKE used this emblem on his flag and fought against Takauji ASHIKAGA until Takauji at one time was driven to take his own life in an instant.
After the period of the Northern and Southern Courts and during the Sengoku period, Akiie KITABATAKE came to be known as a great commander through books such as 'Taiheiki' (The Record of the Great Peace), 'Baishoron' as well as 'Jinno shotoki' (Chronicle of the direct descent of gods and sovereigns), which was written Akiie's father, Chikafusa KITABATAKE. Thus, it is considered that Shingen made his battle flag by making reference to the Fu-Rin-Ka-Zan battle flags of Akiie KITABATAKE.
Further, Shingen believed in the divine protection from the Suwa-taisha Shrine which he had faith in, and used the emblem of 'Namusuwananguhosshokamishimodaimyojin' on his battle flag for the headquarters.
This episode is a famous anecdote of Shingen TAKEDA, and for this reason or another, there are many works that makes Fu-Rin-Ka-Zan as its subject, or those that quote its names.