You can read chapter 15 on my website here.
Or you can read it on on AO3 here.
Here's a quick excerpt:
A few seconds before his boss was supposed to arrive, Sans teleported to his Hotland sentry station.
The moment he materialized, the sound that greeted him made him almost wish he’d stayed away.
“Come on, is that all you’ve got!?”
As Undyne’s strained, struggling voice drifted through the cavern and washed across him, grim resignation pulled Sans’ eyelids closed.
The kid might actually be killing her.
I’d hoped they were past that.