Onimusha: Warlords, produced by Keiji Inafune, is the first game of the Onimusha series. It went through many changes and was released as Genma Onimusha for the Xbox. The game was also ported to PC Windows as Onimusha: Warlords, although this version was only released in Asia. Also Onimusha: Warlords was originally going to be for the PlayStation, but since the release of the PlayStation 2 (Onimusha: Warlords was about 50% done at that time), it was moved to that home console. Artwork was done by Daigo Ikeno.
- Yumemaru - Yumemaru is a young boy that was found wandering the dangerous woods surrounding Inabayama Castle. Princess Yuki has developed a fondness for him, and has adopted him as her brother. When Samanosuke first encounters him, agents of Nobunaga are hunting Yumemaru. Ten years later, he returned under the name Keijiro in Blade Warriors.
- Yuki - The tender and kind-hearted Yuki is the sister of Saito clan leader Yoshitatsu. She has always been fond of Samanosuke, since the days when the legendary samurai served under her father. While her heart broke to watch Samanosuke leave so soon after her father's death at her brother's hands, she has envied the proud warrior for being able to leave Mino province and see the world. She herself cannot, since her hand in marriage has been promised to another vassal since she was very young. In a sense, Yuki has been a prisoner all her life. Throughout the years Samanosuke has been in exile, Yuki has kept up with his activities through her brother's agents, and by contacting his uncle Mitsuhide on occasion. Now that a frightening darkness has fallen over Inabayama Castle, strange events are occurring and Yuki is fearful for her life. She writes a letter to Samanosuke begging him to return, and asks Mitsuhide to deliver it so that the faithful warrior might come to her aid. Since Yoshitatsu is busy planning for battle with Nobunaga and is often away from the castle, Samanosuke is the only other person she can rely on.
- Osric - A gigantic red demon from the Dark Realm who is dispatched to kidnap Princess Yuki, and to stop Samanosuke from interfering in the plans of the demons. Osric returned as a boss in Blade Warriors (misspelled as "Ozrick").
- Hecuba - A giant purple and green-striped wasp woman who lays larval eggs, hatching deadly orange arachnid-like children called Hecubites. When the mother bug attacks, a well-timed block will cause her to bounce off Samanosuke and unintentionally slay her own offspring that are in close proximity. Since she is a flying boss, a wind attack from the Shippuu is the best strategy. Hecuba returned as a boss in Blade Warriors.
- Gate Keeper - Also known as Mino Old Man, he is a mysterious tiny man whose appearance is similar to a bagworm moth larva. He drops from the ceiling and is wrapped in a cocoon, somehow being able to appear in different places throughout the game. He takes a liking to Samanosuke, forever inviting him to various levels of the Dark Realm, a dangerous place swarming with strong Genma. He continued this role in the third game.
The game features samurai Samanosuke Akechi, modeled after actor Takeshi Kaneshiro, determined to save Princess Yuki of the Saito clan and defeat Nobunaga, resurrected after signing a pact with demons. The game also features Kaede, an expert female ninja who provides Samanosuke with information. Kaede was originally sent to assassinate Samanosuke by the Iga ninja clan, but for reasons unknown she did not kill him, and develops trust and deep feelings of love (as shown in Onimusha Blade Warriors) for the samurai and agrees to join him as his confidant. Samanosuke travels throughout all of Inabayama Castle, where he searches for clues of Princess Yuki's whereabouts, finding special weapons and solving various puzzles.
The story begins with a battle between the forces of Yoshimoto of the Imagawa clan and Nobunaga of the Oda clan in a huge field. Samanosuke is seen overlooking the battlefield on top of a mountain when some Oda soldiers come to take his life. However, the soldiers are no match for Samanosuke's amazing swordsmanship and superior fighting skills. As victory seems imminent, Nobunaga laughs until a stray arrow pierces his throat. Afterwards, Samanosuke receives a letter from Princess Yuki of the Saito clan. In the letter, Yuki says that maids and servants have been disappearing and she suspects monsters to be the culprit. She also request that Samanosuke save her from the castle, before she ends up as one of the victims. Samanosuke and Kaede arrive at the castle too late to rescue Yuki. The player then sees an army of soldiers marching with a pale, but living, Nobunaga leading them.
Samanosuke and Kaede split up and head to the keep. After a little ways, Samanosuke finds two ninja Genma and an unconscious Princess Yuki. Samanosuke is unable to defeat them before they escape. As Samanosuke and Yuki have a brief reunion, a gigantic club-wielding Genma named Osric emerges from the ground. Samanosuke attempts to defend Yuki, but is easily dispatched and sent flying against the wall. As he lies unconscious, he is visited by twelve gods of the Oni clan. They give Samanosuke the power to destroy the Genma and absorb their souls with a mystical Gauntlet. As he enters a cave, he finds the thunder orb and the Raizan. As he finds the entrance to the keep, he realizes it is sealed and doesn't have the power yet to open it. In a dry moat, he finds and kills Osric. As it dies, it destroys a part of the wall which leads to a hallway. In the room at the end is a laboratory with a Genma scientist named Guildenstern inside, operating on a human. Guildenstern reveals that he resurrected Nobunaga, and that he made a pact swearing his allegiance to the Genma. Guildenstern then escapes and sends a creature called a Reynaldo against Samanosuke. He defeats it and obtains the fire orb and the Enryuu in the laboratory. With the means to enter the keep now, he goes inside to find the entire place a derelict. Within the keep, he encounters a man named Tokichiro Kinoshita wrestling with a small boy. After the boy escapes, Tokichiro states that he is a servant of the Oda clan and wonders if Samanosuke would be interested in joining. Being a ronin samurai, Samanosuke refuses, saying that he controls his own life. Tokichiro says he will not give up, and leaves the room laughing. As he moves on, Samanosuke sees Kaede chasing the young boy. Samanosuke only has time to learn his name is Yumemaru, before he escapes again and Kaede goes after him. Kaede later learns that he is an orphan, and Yuki has raised him like a brother.
As Samanosuke nears the top of the keep, he sees Yumemaru in the hands of a ninja Genma. Before he can give chase, Tokichiro confronts him once again with an offer to join the Oda clan. He also reveals that something called the Dark Ceremony will take place soon. In it, Princess Yuki's skull will be filled with her own blood. Fortinbras the Demon King will bless it, and Nobunaga will drink it in order to become powerful and destroy the Saito clan, all while offering human sacrifices to the demons. When he finds the room Yumemaru is in, Samanosuke realizes he doesn't have the power to unseal the magical barrier. He fights a strong Genma named Marcellus on the roof, and receives the wind orb and the Shippuu when it is defeated. With this, he is able to rescue Yumemaru and brings him to a secure room in the keep. As Samanosuke tells him about the world, a maid comes in saying that Princess Yuki is underground. Samanosuke tells Kaede to stay with Yumemaru and the woman while he looks for Yuki. While he searches, he finds himself once again in the presence of Tokichiro. With one last request to join the Oda clan, he activates a device and Samanosuke is pulled down into the earth. Meanwhile, Kaede is rendered unconscious by a Genma which looks like Samanosuke, and Yumemaru is kidnapped by a woman. When she awakens, Kaede is led to the prison and finds Yuki locked in a cell. Before she can free her, Guildenstern arrives from the darkness of the cell and leaves Kaede to die at the hand of a powerful Genma. She escapes, once it is dead.
Samanosuke awakens, and kills his counterpart in the underground passage. He makes his way back into the keep and arrives in the room where the woman and Yumemaru are. The woman reveals her true form which is that of an insect-like Genma called Hecuba, and erects a door to the demon world which she flies into with Yumemaru. When Samanosuke faces her, she reveals that Yumemaru is to be killed in front of Yuki which will heighten her sorrow, thus granting Nobunaga even more power when he uses Yuki's skull (the Grail of Despair) to drink her blood. Samanosuke and Kaede eventually vanquish the insect Genma. As Samanosuke makes his way through the demon door, he encounters Guildenstern who summons a more powerful version of Marcellus. Marcellus now has a red pigment all over his body, and some kind of crystal embedded in his chest. After defeating him, he makes his way into Fortinbras' throne room. There he finds Yuki and Yumemaru trapped on the upper level. Before Samanosuke can free them, Fortinbras enters the room. After a brief conversation, the two clash. It would seem that Samanosuke killed Fortinbras, and he frees Yumemaru and Yuki as Kaede enters the room.
As they exit, the place comes crashing down and Fortinbras finds the strength to grab Samanosuke. Kaede, Yumemaru, and Yuki escape as the strain of Fortinbras's grip causes Samanosuke to spit up blood. Some of the blood falls on the gauntlet and Samanosuke becomes a true Onimusha and kills Fortinbras by stabbing him in his central eye. As Samanosuke transforms back into a human, he sees Nobunaga's reflection in Fortinbras' eye. The two stare at each other, as the room continues to collapse. During the ending sequence, Yuki and Yumemaru follow Samanosuke’s advice and travel the world. After the end credits, Samanosuke is seen alive, viewing Inabayama Castle from afar, still possessing the gauntlet. Kaede spends 10 years looking for Samanosuke, until she finally finds him (as seen in Onimusha Blade Warriors).
Genma Onimusha Edit
Genma Onimusha, released for the Xbox in 2002 (a year after the original version), contained many changes from the first release. Besides the requisite minor graphical upgrade, there were various changes to increase the difficulty. Monsters are stronger and there are generally more of them throughout the game, but the most significant addition is green soul energy. Some foes release this when they die and the player must capture it, before other monsters do. The player can gain temporary invulnerability, but the monsters become faster and stronger. Additionally, there is a new area, new cut scenes, several new costumes, enemy limbs can be cut off by the player's weapon on certain conditions, and a new super attack.