Tachi: Kaiten
Tachi: Kasha isn't really the final Samurai weapon skill. Another WS, Tachi: Kaiten is also available, but only if you have one of two level 75 weapons equipped.
Totsukanotsurugi
The Totsukanoturgi is extremely difficult and lengthy to obtain, requiring you to perform multiple upgrades on a Great Katana that's dropped in Dynamis - Bastok and Dynamis - Beaucedine, the Ito. If you do get your hands on one, you'll be able to use Tachi: Kaiten in Dynamis… but only in Dynamis.
Amanomurakumo
This is the most powerful, best of the best Samurai Great Katanas in the game, and it lets you perform Tachi: Kaiten anywhere. It's also the most difficult to obtain, and requires that you upgrade the Totsukanotsurugi.
The Definitive FFXI Samurai Guide
Samurai Job Quest | Great Katanas | Samurai Equipment | Samurai AF | Samurai Job Abilities | Samurai Job Traits | Samurai Subjobs | Samurai Food | Two-Hour: Meikyo Shisui | Weapon Skills | Samurai Merits
Get the most out of your Samurai!