Where's Tonbokiri (The Best in The East's suit set)

I bought Mikuni Guro at the broken blade shop, and now I want to get the Tonbokiri for the suit set effect
I had to forge it, but it's locked with the treasure pavilion, I checked every day, but there's no scheme to forge it, I checked at battlepass and achievement, but none of them are available
Any idea where I can get the Tonbokiri, thank you