The area itself is very small and mostly linear, with only a few enemies, half of which will not even attack unless provoked.Nonetheless, there's some gorgeous views, decent loot, and plenty of frustration in store for you.
First of all, thank you very much for your reply and your help Millennium Master !
He becomes available after leaving and reentering to the Cavern of the Stolen Fayth after completing its events. Even if he has already been summoned, the party may still opt not to fight him and leave using the teleporter. The player can exit and enter the Cavern of the Stolen Fayth and fight Dark Yojimbo as many times as they wish, as long as they do not defeat him for the fifth time in a consecutive order.
Doing so will allow easy Dark Matters, Master Spheres (Master Spheres are a rare drop), Ribbons and/or Break HP Limit to drop every time he is defeated.
It is here that the Primordial Crystal is housed, which gives Seath his immortality and incredible magic power.
It is the ultimate proving grounds of sorcerers everywhere.