So SwapOut is Swap Papyrus, who is stuck in genocide route then he uses the machine thing and somehow gets into Undertale, Which meant Swap Pap sees Swap Sans, UT Sans, and UT Pap died

Swapout by ZKCats
Undertale: Disbelief by Flamesatgames
Underswap by Popcornpr1nce

Undertale by Toby Fox