According to "Nichiren, Leader of Buddhist Reformation in Japan"...The people with him started chanting "the Stanzas of Eternity" from the Lotus Sutra. Now according to the book;
"...When those who imagine that I am extinct honor my relics with great reverence, and feel sincere love for me with unbound piety and adoration, and become so gentle and devout, upright, mild, and resolute in faith, and aspire to see me heart and soul, even at the sacrifice of their lives, then I, with my train of disciples, shall reveal myself on the vulture peak, and tell them the following words: 'I am eternally here, have never passed away'....."
Quietly, gently, the weary master's eyelids closed; and he passed peacefully away.
...thou, having wisdom of it all, Shall never doubt it. Leave all uncertainty forever. My words are all true."
The book also says that his last few days were spend with his 6 senior disciples and other people, among other things, instructing them how to carry on after he passes.