This is an alternate Universe where all the Hashira have become Demons, under the Demon King, Muzan. This includes:

Shinobu: The Butterfly Demon – Lower Moon 2 Mitsuri: The Love Demon – Lower Moon 3 Tengen: The Sound Demon – Lower Moon 1. Obanai: The Serpent Demon – Upper Moon 6. Rengoku: The Flame Demon – Upper Moon 5. Muichiro: The Mist Demon – Upper Moon 4. Giyu: The Water Demon – Upper Moon 3. Sanemi: The Wind Demon – Upper Moon 2. Gyomei: The Earth Demon – Upper Moon 1.

Similar to the events of the original universe, the Demons home is inside of the Infinite Castle, a sprawling environment that has been given a shape by Muichiro, in his daydreaming state.