For the sake of some accuracy, I should clarify few points.
Firstly, Nichiren himself did not claim to be a Buddah. However, he did accuse Zen and Pure Land of be heretics.
Nichiren sect together with Zen and Pure Land are parts of new Buddhist movements started in 12 century. One coule even say that Japan become a buddhist country only after these groups brought Buddhism to ordinary Japanese. Previous Buddhist schools were mainly for nobility class and were mainly concentrated near Japanese capital.
This may be an extrem simplicfication but they did tackle one issue which was problem with Oriental Mahayanan buddhism, that is the massive quantity of sutras and commentary which the previous 1000 years has produced. Zen and Pure Land distanced itself from sutras, Zen concentring on meditation while PureLand focusing on chanting. Nichiren basically decided to focus on just one sutra, Lotus Sutra, as the manifestation of supreme Dharma. Nichiren in turn criticise Zen and Pureland for the lack of Dharma which isn't too unfair criticism given that they move away from Buddhist cannons. You might consider Nichiren as Japanese version of Luther.
Anyway, Nicihren had 6 diciples and after Nichiren dies, there is a split between the leader and the rest of 5 diciples. The Nichiren sects formed by these 5 diciples are called Nichiren Shu and they are, in general, quite o.k. The other one which was set up by the leadercalled Nichiren Shoshu. Nichiren Shoushu consider Nichiren as Buddah of Dharma ending era. According to Nichiren Shoushu Triple gems are Nichiren (Buddah), Lotus Sutra (Dharma) and Nishiren Shoushu (Sangha). They accuse every other buddhist sects of heresy destined to hell. Consequently, they were banned during Tokugawa Shogunate as a cult. Sokka Gakkai is a lay organisation of Nichiren Shoshu which was excommunicated about 10 years ago.