Pokemon has been an amazing, huge influence on my life, for better and for worse. I never thought another game was going to have the same impact on me as the Pokemon series had - until Undertale came along. Between Pokemon, Undertale, and a major thing that happened a couple years ago, I started kicking around the idea of a crossover Undertale battle. I wasn't exactly sure how it would work...until I saw UnicornArin's custom fight (at http://unycorn.itch.io/undertalearin .) Arin was kind enough to allow me to be his first commission for an Undertale fight - and after a month or so of planning specifics, an entire overhaul of the text, and more conditionals than I ever realized (sorry Arin), my custom fight is live!
I would like to thank LadyLightest, SSJ3Mewtwo, Dogurasu, Nonine, CtrlAltDel123, and anyone else who helped with any of the planning. Dogurasu gets the credit for working out about 80% of the text, and everyone else with text suggestions and playtesting to find bugs.
Non-spoilery bits: There's a way to spare, and a way to make killing easier. Arin's fight gave me a fairly concrete idea on how I wanted to handle both of these things and you'll see that there's a pretty consistent parallel between the two. And in general, if you think something's a reference to something, it probably is. Undertale is chock-full of double meanings and I wanted to reflect that I enjoyed that in my own fight.
I may post all of those double meanings later once enough time has passed.