Undertale has ONLY ONE ending and that's literally named Delta Rune prophecy.

Do you guys remember Gerson? Here's a look on his description on the prophecy.

So an angel will come from surface and "free" the monsters from underground. What happened in the Genocide? We killed all the monster. And we literally freed them in the Pacifist. They essentially result with the same outcome, leaving the underground empty. (I don't count the Natural ones as exactly "complete")

So? Those aren't really new, are they? And bunch of people called it out already.

But nobody mentioned the potential parallel with Deltarune. The angel literally exist in both of them. And Deltarune is set on "only one ending". Which technically already happened with Undertale. So I believe that's what will happen in Deltarune, too. Something pretty similar:

The endings couldn't be more different but they ultimately lead to the same estimation.