I'm not familiar with Undertale and not usually into monster girls, so when I first started reading this I kinda dropped it midway. Somehow it felt too long too.
But now I've stumbled upon this again on archiveofourown and saw it was only 4 chapters and the last seemed to be about a human girl, so I decided to skip right to it… and it was awesome! Especially the very last part where they raise children just to end up snuffing them too, but are still genuinely happy to live as a family like that, while also anticipating when he would snuff them all again, and how the memories of snuffing his beloved all the previous times come to him as happy and warm ones. Ah, this is just so hot and sweet and everything!