I read something about a portrait of Sai. What's the story behind it and why did it take so long for the full story to be told?

Chapter 41 recounts a story about a portrait of Sai. The narrator explains that they and a priest named Ismu made a firm determination to present the powerful portrait. The narrator presented the image but had to leave quickly as Sai's meal was ready, leaving no time to explain the full history behind it. The narrator intended to tell the story later, but as they put it, while thinking "today or tomorrow," a full nine years passed before an unexpected meeting occurred, finally providing the opportunity to recount the past events and connect them with a wonderful dream the listener had.