Okay so, I've been thinking, what if undertale and deltarune are just well that, they don't connect they don't met, they don't link, what if they are just to different games and two different universes, what if they are just a parallel story. What if sans is just sans, not deltarune sans, just two different sanses, what if gaster just straight up died or got the dess treatment in undertale(Being trapped in the code), what if DR gaster is the man behind the tree and the voice at the beginning of the game and isn't from deltarune. I know that there's a lot of evidence for the other side of this coin and I genuinely want it to be this to not be true, but I'd say that there is more evidence for the two games being to different universes not connect by others like sans or gaster. Plus if it was true it would create some very weird and confusing timeline stuff, like you got sans and papyrus appearing while gaster was still there from deltarune but deltarune was created by gaster in undertale so how does that work. It's just more logical to think "Okay these are two different games in two different universes", and that's okay, we don't tricky tony to go all out MCU here, we have two near perfect games and it's okay if they aren't a prequel or a sequel to the other, they are as toby said, Parallel stories...
YES YES YES YES YES YES YES YES THIS THIS EXACTLY
I mostly agree
though I do truly feel that there is SOME connection (namely just sans)
but i do think that you don't need to see them as prequel/sequel, or even connected
merely parallel stories, as you (and toby) have said
the way I personally see it is like the knives out films
with sans as blanc
he's the same guy, he's there in both, he's seen both, but it's like whatever man nobody cares
I mean, I agree with this, with one minor connection of Sans Deltarune going to Undertale somehow at some point. Toby himself said that they're sister games, not a sequel[1] and a first game[2]
Right, @Fwugradiation?
I fw ugradiation
If UT and DT have zero correlation, then Gaster would be a normal person in Deltarune, not a cryptic type shit or a nonsensical character
Undertale and Deltarune are connected at least with ENTRY NUMBER SEVENTEEN.
In Undertale's room_gaster, in Wing Dings:
ENTRY NUMBER SEVENTEEN
DARK DARKER YET DARKER
THE DARKNESS KEEPS GROWING
THE SHADOWS CUTTING DEEPER
PHOTON READINGS NEGATIVE
THIS NEXT EXPERIMENT
SEEMS
VERY
VERY
INTERESTING
...
WHAT DO YOU TWO THINK
In him.png (which is an image that was on deltarune.com long before Deltarune came out), also in Wing Dings:
THIS NEXT
EXPERIMENT
SEEMS
VERY
VERY
INTERESTING
Some time later, but still before Deltarune came out, him.png changed to:
THREE HEROES
APPEARED
TO BANISH THE
ANGEL S HEAVEN
I have no doubt that Undertale's ENTRY NUMBER SEVENTEEN is about Deltarune.
Oh yeah, forgot about that
Am I the only one who finds rhetorical questions exhausting?
But that's impossible! Think about all the years of theory videos! And the place where it rained! And the scratched off "Grillby"! - some random annoying guy who's definitely not me
Yes
I don't disagree with this, i just said that if one is to agree to this theory, that person would believe Gaster is a common character in DT.
There's a way to explain that last one, sans bought the store and it was gribbly's he was just to lazy to buy a new sign. That second one being the place where it rained, that's a little harder, but we all know that it's just the It's rain somewhere else but without the sans motif. Well we also know that undertale sans is aware of the other timelines, so this is the place where it rained for another sans.
Okay so my theory is just that with deltarune gaster, that we was once a normal townie, who kept to himself and maybe hung out in the bunker darkworld experimenting with darkness, then(you'll need to read my angel theory for this) he was able to bring back a small peice of that deepest dark and brought back FRIEND, then he logged his experements with friend and mabye dess, then he wrote entry number seventeen when he foundout that friend was planning to cause a world ending event. who then to have a little fun banished him to another dimention, where everyone forgot...
Well now explain how entry number 17 is in UT
I agree, but in a different way. Undertale gaster was experimenting with multiverse theory, when he found Deltarune, after he kept an eye on the world of deltarune he choose to mess around with it, which is where the voice comes in, but not the voice at the beginning of chapter one, he is the voice at the end of chapter four,
WELL DONE
THE HALFWAY POINT HAS BEEN ATTAINED
BUT IT IS STILL WAITING
HOW MUCH LONGER NOW?
MY... DELTARUNE.
he says this to the play, congratulating them on getting closer to bringing the deltarune version of gaster back due to friend banishing him and then it was that which caused the undertale version of him to become lost in the fun events, the shadows cutting deeper is just some of that darkness that the dr gaster is imprisioned in leaking out into other worlds, which is why neither is perecftly gone
the games are actually a lesbian couple
Yuri 
“I gently open the door…”
Guess who, idiots.