The castle lord, Matsui Yoriyuki, had written Hosokawa Mitsunao, lord of the Hosokawa Clan, to protest being denied the military rank held by his family for generations, including his father, who had just announced his retirement.
Historians aren't sure why Yoriyuki was denied the promotion, "but it must have been important enough to his family to risk dying for," said a museum representative.
Lord Hosokawa must have been impressed with this display of courage, because Yoriyuki not only survived but later went on to serve as karo, a top-ranking samurai official, in the shogunate.
A photo of the actual will is shown below. You can see a close-up of another piece of Yoriyuki's writing here.