Yakozen is an evil kind of zen that resembles real Zen but is completely different from the real thing. The word appears in the 'Hyakujonoyako,' the second rule of 'Mumonkan' (The Gateless Gate; a collection of 48 Zen koans compiled in the early 13th century by the Chinese Zen master Wumen Huik'ai). A "yako" is a low level yoko (fox spirit). It is also called "yakozei," "yakoshin" and "namazen."
As seen in the phrase 'Buppo (the principle of Buddhism) is in muga selflessness,' the true Buddha should be sensed through selflessness after emptying the ego. A Yakozen refers to a self-satisfactory taigazensha (one who practices zen but has an enormous ego), who uselessly proclaims they have earned shokaku (true enlightenment) even though there is no proof, also called "misho-isho." It is one of the so-called mazen (evil zen).
In a koan (a Zen question for use in meditation) within the 'Hyakujonoyako,' the second rule of 'Mumonkan,' there is a story about an old man who lived in Mt. Hyakujo in a previous life who claims that those who have once reached enlightenment do not 'fall into inga (cause and effect; karma).' Instead, by concentrating only on a state of 'ku' (emptiness) and ignoring the principle of inga, they fall into a state of chikushodo (the realm of animals) as a yako for five-hundred lifetimes. In this story, a caution is made through an example of self-satisfactory conceited zen, that by being caught in a mental state of 'ku,' it is possible to mistake that one has reached enlightenment, even though they have not reached the state of true myou (mysterious or wondrous manifestations) and myoyu (mysterious influence; mysterious effect).
Nichiren harshly criticized a Zen sect in one of his shika-kakugen (four criticisms by Nichiren against other sects of Buddhism) that they were 'zen tenma' (evil spirits of zen). Nichiren said that the reason for the criticism was due to the Zen sect not being based on Buddhist scriptures and having the principles of kyouge-betsuden and furyu-monji (both meaning that the principles are inside one's heart, not on scriptures), but it is believed that his criticism was on the aspect of a person possibly falling into yakozen.