Development Discussions and concept work for Oninaki began in 2017, during the last stages of development for Lost Sphear. Oninaki was the last of three titles conceptually planned by Tokyo RPG Factory when it was founded to work on I Am Setsuna (2016). The basic plan was for a game built on the resources of the previous two games. A notable addition to the staff was Takashi Tokita, who had gained fame for his work on Chrono Trigger, Live A Live and Parasite Eve. Having previously provided creative input for the battle systems of its earlier titles, Tokita decided that this time he wanted a deeper creative role. Tokita was a general overseer for the project as creative producer, as well as helping create the basic world view and scenario. Main producer Ryutaro Sasaki, who had a supporting role in the company's earlier titles, supervised the gameplay and technical side of production. Atsushi Hashimoto returned as director from Lost Sphear and I Am Setsuna, as did scenario writer Hirotaka Inaba. Oninaki used the Unity game engine. The game's earliest concepts included the continued use of the turn-based combat system from I Am Setsuna and Lost Sphear, with Kagachi fighting alone to protect the young Linne. Tokita wanted the studio's games pushing for reinvention, something he compared to the drastic gameplay changes between Final Fantasy and Final Fantasy II, which later led to the SaGa series. There was internal opposition to the change at first, but Hashimoto brought the staff round to the idea. Hashimoto was also exhausted with the turn-based system. Hashimoto wanted the ability for Kagachi to switch jobs, a term referring to schools of skills characters can learn such as magician or warrior. Hashimoto also wanted the character to be able to switch between jobs in real-time, which contributed to the shift from turn-based to action-based gameplay. The shift between the real world and the Beyond started as a joke about moving between the worlds of I Am Setsuna and Lost Sphear, but eventually became one of the founding elements of Oninaki. The game's dualistic story themes were derived from this style of shifting gameplay. Sasaki was initially aiming for a 20-hour experience, but Tokita pushed for a longer campaign, so it was eventually extended to past 30 hours. The Daemon system, compared by Hashimoto to the job system of Final Fantasy V, was designed so players would