Uninhabited Sekai is 25-ji, Nightcord de.'s sekai, though initially created by just Mafuyu.
It is accessible via the song "Kuyamu to Kaite Mirai".
Kanade, Mafuyu, Ena, Mizuki, Miku, Rin, Len, Luka, MEIKO and KAITO have conversations here. Additionally, area items that increase the power of 25-ji, Nightcord de. and VIRTUAL SINGER cards can be found here.
Areas
Main
The main area of the Uninhabited Sekai is automatically unlocked straight away and was previously the only visible section of the Sekai from the World Map. It contains area conversations as well as the area items shop where players can purchase and upgrade items that increase the stats of specific character cards.
Lake
The lake area of Uninhabited Sekai is unlocked by reading every episode of the "Searching for a Reflection Beneath the Waters" event story - which is also the story it is first introduced in and discovered by the 25-ji, Nightcord de. members. It is an extension of the Sekai based on the growing feelings of the members.
As opposed to the main area of the Sekai, the lake doesn't have any area items, nor an area item shop, but it does have its own area conversations between the members.