Onimusha: Warlords was originally being developed for the original PlayStation, but the project was eventually moved to the PlayStation 2. The project was originally intended for the Nintendo 64's 64DD. According to Okamoto, the house would contain 'hidden doors behind walls, ceilings that fall down to, scrolls and ninja magic, and many other ninja techniques'. The series originated in 1997 when Yoshiki Okamoto had an idea to create Sengoku Biohazard, a ninja version of Capcom's 1996 survival horror game Resident Evil (known as Biohazard in Japan), set in the Sengoku period and featuring a 'ninja house' filled with booby traps, similar to the mansion from Resident Evil, where battles would be fought using swords and shuriken. Jissen Pachi-Slot Hisshouhou! Onimusha 3 - (2005 - Japan only) - a pachislot game based on Onimusha 3 developed by Sammy Studios and published by Sega.

These magical attacks, which vary depending on what weapon is equipped and other offensive attributes, can be raised throughout the game by accumulation of souls from defeated enemies or certain items or trinkets. The player does, however, possess a limited supply of spiritual energy which can be used for magical attacks. The player uses a various arsenal of destructive weaponry, ranging from regular katana to elemental-based broadswords.

Unlike most of the mainline Resident Evil games, Onimusha is very action-oriented with a heavy emphasis on combat. What may frustrate some gamers is that Onimusha's characters cannot perform basic actions that have been taken for granted in most other action-oriented titles, such as jumping, grabbing, and climbing over obstacles. Characters tend to move slowly and can only slightly increase their speed with the dash maneuver by tapping twice in any direction. The player controls their character using the D-Pad (although later games such as Onimusha 3: Demon Siege and Onimusha: Dawn of Dreams introduced analog stick control) and travels in a fairly linear method, able to rotate slowly with the input of an opposing direction. The control scheme of Onimusha resembles that found in the Resident Evil series. In each game, the protagonist has the ability to absorb Genma souls from defeated enemies, which helps to restore health, infuse power within weapons and armor, and provide power for the elemental attacks of special weapons. Even though the protagonist changes in every Onimusha title, he is always a skilled swordsman who embarks on a set mission and journey, which involves slaying demons and fearsome enemies during the waning years of the Warring States period in feudal Japan. The gameplay in most of the Onimusha games is roughly based off of the mechanics from Capcom's survival horror franchise, Resident Evil.

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