What is the last prize in Kuji?

I'm curious, could you please explain to me what the final prize is in the context of Kuji? Is it a particularly valuable or sought-after item, or does it vary depending on the specific Kuji event? Additionally, could you elaborate on the mechanics behind earning or obtaining this last prize? I'm eager to learn more about this aspect of Kuji and how it adds to the excitement and anticipation of participating in these events.