Bill's Dimension is the home dimension of Bill Cipher, introduced in The Mysterious Seven Project. It appears to be an abstract forest of sorts, with eyes floating to reveal other demons, some nearly identical to Bill in appearance.
Logic and facts seem to be abandoned in this other dimension.
This stage is "created" as the characters stand, and only the characters. Assists will fly off as well as items unless another player is nearby. Occasionally, a demon will pop out and shoot a colored fire at players. A player can either be knocked out of the stage or be grabbed by a demon's arm.