Day 17 covers all of Chapter 12, Devadatta, and opens Chapter 13, Encouragement for Keeping this Sutra.
Having last month heard Śāriputra’s objection to the daughter of the dragon-king becoming a Buddha, we witness the daughter of the dragon-king become a Buddha.
At that time the daughter of the dragon-king had a gem. The gem was worth one thousand million Sumeru-worlds. She offered it to the Buddha. The Buddha received it immediately. She asked both Accumulated-Wisdom Bodhisattva and Venerable Śāriputra, “I offered a gem to the World-Honored One. Did he receive it quickly or not?”
Both of them answered, “Very quickly.”
She said, “Look at me with your supernatural powers! I will become a Buddha more quickly.”
Thereupon the congregation saw that the daughter of the dragon-king changed into a man all of a sudden, performed the Bodhisattva practices, went to the Spotless World in the south, sat on a jeweled lotus-flower, attained perfect enlightenment, obtained the thirty-two major marks and the eighty minor marks [of the Buddha], and [began to] expound the Wonderful Dharma to the living beings of the worlds of the ten quarters. Having seen from afar that [the man who had been] the daughter of the dragon-king had become a Buddha and [begun to] expound the Dharma to the men and gods in his congregation, all the living beings of the Sahā-World, including Bodhisattvas, Śrāvakas, gods, dragons, the [six other kinds, that is, in total] eight kinds of supernatural beings, men, and nonhuman beings, bowed [to that Buddha] with great joy. Having heard the Dharma [from that Buddha], [a group of] innumerable living beings [of that world] understood the Dharma, and reached the stage of irrevocability, and [another group of] innumerable living beings [of that world] obtained the assurance of their future attainment of enlightenment. At that time the Spotless World quaked in the six ways. Three thousand living beings of the Sahā World reached the stage of irrevocability, and another group of three thousand living beings [of the Sahā-World] aspired for Bodhi, and obtained the assurance of their future attainment of enlightenment. The Accumulated-Wisdom Bodhisattva, Śāriputra, and all the other living beings in the congregation received the Dharma faithfully and in silence.
On the topic of the daughter of the dragon-king becoming a Buddha, Nichiren writes in Kitō Shō, Treatise on Prayers:
Speaking of attainment of Buddhahood by this female dragon, therefore, Grand Master Miao-lê writes, “The great blessing despite little practice shows the power of the Lotus Sūtra. “As this female dragon is indebted to the Lotus Sūtra for attaining Buddhahood, how could she abandon practicers of the sūtra even without the Buddha’s command? So, she says in verse praising the Buddha,”I will widely disseminate Mahāyāna Buddhism to save the suffering people.” This vow of the female dragon is at the same time the vow of all the dragons who follow her, which is so deep that the mouth cannot express it and the mind cannot fathom it. The Sāgara Dragon King, father of the female dragon, was a beast in body but had a deep compassion for his children that he gave the greatest treasure in the ocean, a wish-fulfilling gem to his daughter as a donation upon her attainment of Buddhahood with the present body. This gem was worth as much as all things in the whole world.
Kitō Shō, Treatise on Prayers, Writings of Nichiren Shōnin, Faith and Practice, Volume 4, Page 63-64