just listen Space to see his phase 1 design
Firstly, yes. it is Spooky's Jumpscare Mansion mixed with Undertale. anyway, the lore dump for this AU. After the war between Humans and Monsters, Most of them were sealed into The Underground, however, Sans and a few others evaded the event, living as folk tales kinda like big foot, eventually Sans enters Spooky's jumpscare mansion, to which he is offered an opportunity by Spooky, a permanent place to stay. Sans takes it but eventually starts having hallucinations of a killer human entering and trying to kill him and the other experiments, since he sees them as family due to his isolation. eventually a human enters the mansion and if they use the axe a lot, he'll attempt to stop you, wielding a spare knife he found. some fun facts about this AU. Sans's jacket has the marks due to him having to patch up his jacket and shirt after the war ended. where you fight him in the mansion is his own designated floor, normally this floor would be a chill area where you can just talk to sans.