In 16th century Japan, the African slave of a Jesuit priest rises to become a samurai in the service of its most powerful warlord, and faces the challenge of a corrupt general who is scheming to topple the warlord and rule in his place.

Agree about keeping it simple. Focus on the story hook.

The hook of the story is not the Jesuist priest. ?And it's not having to fend of a competing, corrupt rival (although that's the central conflict). ?The hook, the sizzle in the concept is that a Black man actually rose from slavery to the rank of a samurai in the closed ?and ethnocentric world of Medieval Japan.

The rest is window dressing, detail for the script.

The logline describes a vague plot because the inciting incident "...rises to become a samurai..." seams like a long process initiated by the main character rather than a sudden even which changes the MCs life and forces him to take action. It seams like the "real" inciting incident that motivated him to want to become a samurai would have happened before hand, what was it?

The second big problem is the goal is not clear, what does "...the challenge of a corrupt general..." mean specifically? What will it look like? Best to be specific about the goal and what he must achieve?

Lastly what is at stake? What could make this a compelling story is a clear definition of what he stands to lose if he fails and gain if he succeeds.

Hope this helps.

If you can simplify further, it'd be a big improvement. Made my head spin a bit. Something more like: "In ancient Japan, a priest's slave becomes a samurai, and fights to defend his warlord against a corrupt general."