Board Thread:General Discussion/@comment-26006155-20190617222636/@comment-32182236-20190813111703

The Halloween Hack is supposed to be an alternate timeline, where the children that went back in time to stop Giygas never returned.

As far as I'm aware, that's literally it. Meaning anything that happens before then on the timeline is still canon.

As for Mother 3, I'm told it has some backstory that happens way before Mother 1.. Though the actual events we play through are probably irrelevant in terms of the theory, because Ness already left to Undertale's world by then (unless Ferret would like to argue otherwise.. But note that if Ness waited that long, they'd be an adult by then, not a kid like Frisk...), the backstory might not be irrelevant. Which is why I'm still analyzing Mother 3.