Saisho Mototsune served as a bushō during the Sengoku period.  Mototsune was the lord of Tatsunokuchi Castle in Bizen Province.  He was a landlord of the eastern bank of the Asahi River in central Bizen.

Origins

The Saisho were members of the Takarada clan, an illegitimate branch of the Jōtō clan who descended from the Kibi clan of the Jōtō District of Bizen Province.  Sai is surmised to have been a hamlet in the Takarada township and the clan was based in this location for a long time under the Tō surname.  A location under the name Sai continues to exist to the present time.  At the time, the character of “sai” in his surname had the same meaning as the character “zei” meaning taxes, so his surname was also written with that character.  The provincial headquarters for Bizen was located on the fields spreading to the south below the foothills of Mount Tatsunokuchi in which Motosune’s residence of Tatsunokuchi Castle was located.  Vestiges of the family temple of the Jōtō clan known as the Shōdahai Temple and related ancient artifacts remain on Mount Tatsunokuchi.  Military chronicles written in later eras described the Jōtō clan as an influential landlord in a border area but with various embellishments not ground in historical fact.

Theory of assassination by Ukita Naoie

Mototsune was known as an intrepid bushō under the command of the Matsuda clan.  He halted the encroachment by Ukita Naoie into the western portion of Bizen at a time when Naoie was expanding his power as a member of the Uragami clan, toppling Toishi and Kameyama castles.

In 1561, Naoie assigned Ukita Tadaie as the commanding general of an army to attack Tatsunokuchi Castle.  Upon hearing of the impending assault, Mototsune set-up an encampment at Taketa-Kawahara and, after a major clash, the battle ended in a draw and Mototsune returned to his base.  Tatsunokuchi was situated with steep cliffs on the northern and western sides and the Asahi River flowing in the foothills below.  To the south lay a deep valley and, nearby to the east, stretched a chain of mountains.  This terrain made it an impregnable fortress. 

According to military chronicles, events unfolded as follows:  Having determined that it would be reckless to attempt to capture the castle by force, Osafune Sadachika proposed a plot to Naoie.  He knew that Mototsune had homosexual tendencies, especially toward attractive young males so a decision was made for Naoie to send his attendant, Oka Kiyosaburō (later known as Oka Gōsuke), as an assassin.

On one occasion, Kiyosaburō observed Mototsune stringing a rope on the river.  Deeming this a good opportunity, Kiyosaburō played a flute on the banks of the river to attract Mototsune’s attention.  Mototsune soon noticed him and took Kiyosaburō back to Tatsunokuchi Castle as the plot unfolded according to plan.  Mototsune was drawn to Kiysaburō and sought to have him at his side but retainers who viewed it as dangerous warned him not to do so.  Mototsune, however, was infatuated with Kiyosaburō and enjoyed merrymaking alone with Kiyosaburō and fell asleep.

One summer, Mototsune enjoyed drinking saké in a spot to escape the summer heat that he made along the Asahi River.  As usual, Mototsune fell asleep.  After confirming that no retainers were around, Kiyosaburō removed Mototsune’s short sword and stabbed him in the chest, killing him.  He then took his head and fled.  The retainers soon realized what happened and gave chase but Kiyosaburō evaded them and safely returned to the Ukita, presenting the head to Naoie.  Thereafter, Naoie took advantage of the situation by attacking and toppling Tatsunokuchi Castle which provided an opening to encroach on the western portion of Bizen.

According to historical records, he was assassinated in Tenshō 2 (1574) or Tenshō 3 (1575).

Alternative stories from both camps

Mototsune had dealings with assorted powers including the Uragami, the Matsuda, and the Amago clans.  Despite other stories indicating that he was assassinated in a plot orchestrated by Ukita Naoie, in the end, the Mōri, in the course of their westward advance, appear to have been responsible for the death of Mototsune.  The preceding account of his assassination by which Naoie took advantage of Motosune’s homosexual interests is drawn from military chronicles from later eras containing accounts of questionable authenticity.

Based on a hōkōgaki, or personal account kept by bushi of their duties, presented in 1644 by Tabei, the grandson of Oka Yozaemon who served Naoie, Naoie ordered Yozaemon to murder Saisho Shinano-no-kami, the lord of Tatsunokuchi Castle.  Yozaemon then set a fire inside the castle and in the ensuing chaos among the soldiers, captured Shinano-no-kami alive and then took his head.  He also took some retainers hostage and departed.  The authenticity of this account, however, is uncertain.

Meanwhile, there is a personal account from 1615 detailing the battles experienced by Nakashima Motoyuki, the lord of Tsuneyama Castle in the Gyōbu township.  At the time, he was also affiliated with the Mōri.  Based on his account, in the fifth month of 1561, at Fort Funayama in the village of Hirase in the Mino District of Bizen, Susuki Buzen-no-kami (under the command of the Neya clan) and others stayed as commanders at the base.  Uragami Munekage then advanced to Kameyama Castle to oppose them.  Mototsune, who at this time was aligned with the Uragami, secretly informed Nakashima Kaga-no-kami (Teruyuki) that he colluded with the Mōri.

On the condition of a hostage, Kaga-no-kami had Mototsune give a written pledge to his younger brother, a priest at the Madani Temple, which he tendered so Kaga-no-kami consulted with Ishikawa Hisatomo and put the brother under the watch of Takahashi Uma-no-jō, the lord of Sakazu Castle.  Bitchū forces then entered Tatsunokuchi Castle and had Mototsune guide them to an escape route in the mountains.  Following news that his younger brother escaped, when Mototsune came out to the west side of the castle, Kaga-no-kami signaled Kajiya Hachibei who grabbed and shoved him down into a waterfall, killing him.  

The death of Mototsune was explained to the retainers of the Saisho in the castle on the pretext that Mototsune slipped on the rocks and fell to his death.  These men were talked into coming under his command by saying “There is nothing more that can be done about Mototsune but I want each of you to submit a written pledge that you have no duplicitous intent and will be loyal to those from Aki Province (the Mōri).  I am sure there are no objections.”  An attendant who was cut-down along with Mototsune survived so when Susuki Buzen-no-kami went to check the body and discovered that he was still breathing, he stabbed him to death.  It then became known among persons in neighboring provinces that the lord of Tatsunokuchi Castle was killed by this plot.