Talk to Despachiaire (K-10) on the top floor, through the entrance with Fouagine beside it, and inside the Walnut door at the back. You may need to speak to him 2 times, you'll know you've got it when you get a cut scene involving Tenzen. (Not the one about the Mithran sin hunters). Note: there are multiple Walnut Doors, the correct door is located at (K-9).
WINDOWER ALERT: On July 24, 2019 Windower 4 with Enternity addon and Fast CutScene addon turned on froze/hard locked the game client during this cutscene at Prishe's line: "Let's make that world right here!". Further confirmation and testing needed. I just relogged without Windower at all, using only the basic game client, and the cutscene completed. Some combination of disabling those addons might be all that was needed.
Update: If stuck in the CS while using Windower, open Task-Manager and rightklick on your Game and select "send feedback". That works everytime.
Prishe has returned to Tavnazia, and it would be wise to follow. The winds of twenty years past have once again begun to swirl about the islands. What evils do these winds carry?
SPOILER WARNING: Click here for additional details about this story line.