Takes place after arc 4 or season 2 of the anime. This may not be as lore accurate but it was the best that I could do.
Credit to Tappei Nagatsuki.
Prolouge:
Subaru walked towards the mansion. He was coming back from his training.
"You are finally done with you training, I suppose?" Beatrice questions.
"Yup, I feel like a stronger man!" the dark haired knight exclaims.
"You have to be a man first before you can become a stronger one, I supposed," Beatrice jokes.
"Now that isn't very nice," Subaru whimpers.
The knight and the loli walk towards the mansion. Emilia walks towards the two and waves.
"Hey subaru, how are you doing?" Emilia asks.
"After seeing you my day got much better." Subaru's way of speaking made Beatrice snort. They continue to walk towards the mansion.
"Hey, Subaru," Otto calls out.
"What's up Otto?" Subaru asks.
"I want you to come check something out." Otto gestures towards the mansion. Subaru follows Otto with Beatrice and Emilia following close behind. They all walk towards a room. In the room Garfiel is staring at a corner of the room with a confused expression. Otto points at the corner that the blond demihuman was looking at. There in the corner was a what seem to be an out of place patch. It was a one foot section of the wall. The normally white wall was replaced with spots of dark purple, black, and white all shift around abnormally. A small buzzing sound came out of this patch too.
Otto pointed towards this odd patch, "Do you have any idea on what this could be?"
Subaru took a very close look at the patch. For a moment everything changed. He was in a room with yellow walls and a moist carpet. Subaru rubbed his eyes and everything went back to normal. "I don't know," Subaru says in a worried tone. "I don't think we shoul-" Right before subaru could finish his statement Beatrice had already placed her hand on the weird spot.
Beatrice's hand went through the spot and she almost fell forward. Subaru had caught her before she could fully fall through. He pulled the spirit back. "What the hell was that beako?!" Subaru said in a worried tone.
Beatrice looks at the wall in shock, "How should I have known that I would fall through, I suppose." All of the people in the room just stared in shock at what happened. Unable to make sense of what was happening.
"I think we should leave," Otto says nervously. As soon as he reached the door the whole room changed. It was all now the same as the patch. With that all of the people in the room fell through the floor as all of them fall unconscious.
Garfiel is the first to wake up, the first thing he hears is a weird buzzing sound. He groans as he gets up. He looks down and sees Otto, Subaru, Beatrice and Emilia all out on the ground. Garfiel slowly looks around to see that ehy are in a competely different place that wasn't the mansion. This place had yellow walls and a yellow carpet.