Originally Chan/Zen was the name of a sect in China (Chan) and Japan (Zen) It became a fad with the intellectuals, Samurai and Certain governmental regimes in Japan but originally sprang from various sects in China that had sprung from Tien-tai Sect.
They focused on riddles and strict zazen meditation. Leaving Tien-Tai was a non-appreciative thing to do. The appropriated lots of Tien-tai but missed the most important things. They diminished the importance of the sutras and replaced real buddhist practice with Koan riddles. The most prominent WERE Hakuin and Dogen, who both returned to the Lotus Sutra in later years but somehow most Zen was based on their earlier years and their fictional founder BODHIDHARMA.
Nichiren called zen
, "The bottomless pit" because they fell into the arrogance of thinking they didn't need to practice that they were Buddha's already. Missing a major point. Theoretically all beings are "Buddhas" already but inactuallity it goes much deeper than that.