Nobunaga Oda is the main antagonist in most of the Onimusha series. In real world history, Nobunaga is viewed culturally as both a hero and as a villain for respectively trying to unite his nation during the Sengoku Period and the horrific lengths he took to achieve his goal.
Background
Born Kippōshi (吉法師), Nobunaga was born on June 23, 1534 as the second son of militant governor Oda Nobuhide. After his father's death, Nobunaga was forced to contend to deal with rival factions with his clan to become its head. Nobunaga then faced the army of Imagawa Yoshimoto in the Battle of Okehazama.
Though Nobunaga's troops where victorious, Nobunaga met an untimely death after the battle when he was killed by a stray arrow which pierced his throat. Although he was presumed dead, Nobunaga was resurrected by the Genma who saw potential in his cruel ambition to add him to their ranks.
Nobunaga as the Genma Lord
Nobunaga in Onimusha: Warlords
In Onimusha: Warlords, Nobunaga was set to be initiated into the Genma by the demons' god Folopinbras by drinking the blood of Yuki to obtain the power to wipe out the Saito Clan. But the intervention of Samanosuke Akechi resulted in Fortinbras's death as Nobunaga confronts the samurai after reverting from his Onimusha form. The exact details of Samanosuke and Nobunaga's stand-off is told by Samanosuke himself in Onimusha Blade Warriors.
Regardless of not drinking Yuki's blood, Nobunaga imposed himself as the Genmas' leader as he uses them in his campaign to unite Japan under his banner. In the process, Nobunaga wiped out the Yagyu village, invoking the vengeance of Jubei Yagyu and setting up the events of Onimusha 2: Samurai's Destiny.
Nobunaga exercised his power to force slaves to mine huge supplies of gold in order to build the extraordinary Golden Evil Statue. This demonic behemoth was imbued with Nobunaga's spirit and was intended to crush any resistance, until Jubei summoned his ultimate Oni power to destroy it, along with Nobunaga. But as Jubei did not possess the Oni Gauntlet, Nobunaga's spirit escaped and eventually regained physical form as he resumes his conquest.
Honnoji Temple Incident and After
Nobunaga's end finally comes at Honnoji Temple in 1582 where Onimusha 3: Demon Siege takes place. At that time, Nobunaga arranged for Guildenstern, the Genma who revived him from his mortal injury, to create temporal distortions so the Genma can invade all moment in time. This plan was thwarted by Samanosuke Akechi with assistance from the time displaced Jacques Blanc.
In the end, Samanosuke defeats Nobunaga with the warlord's spirit absorbed into the Oni Gauntlet. After momentarily having Soki use the Oni Gauntlet to access Nobunaga's power during the final battle with a resurrected Fortinbras, Samanosuke and Ako, under the aliases of Tenkai and Airin embark on a mission to seal the Oni Gauntlet away so Nobunaga could never be freed.