THANK YOU FOR WAITING SO LONG. AFTER ALL YOU AND I, WE HAVE BOTH BEEN WAITING SUCH A VERY LONG TIME.
Unmarked spoilers for both Deltarune and Undertale ahead.
On October 30th, 2018, followers of Toby Fox's Undertale twitter account saw a tweet asking them to partake of a Survey Program that, once downloaded, began as a sort of character-creating game. A voiceless narrator who may or may not be W.D. Gaster encourages you, telling you how wonderful the life you are creating is.
However, after you've created your "vessel," the narrator discards it and tells you that this game has no choices. You can't choose who you are, because you are. . .
Kris.
Kris, a human teen(?) living with Toriel in an unspecified area on the surface world, in a town full of monster citizens from the Undertale. Kris, whose high-achieving brother Asriel is away at college, whose father Asgore runs the flower shop, whose mother isn't a queen, but a teacher at the local school that Kris attends.
Thus begins the first chapter of the sequel/spiritual successor/spiritual precursor to Undertale: Deltarune.
What is it?
Deltarune (so far) is the first "chapter" of an upcoming completed Deltarune RPG game. Though it is only a chapter, it can be considered a complete game of its own, in that there is a straightforward, fully-fledged story that you can finish. It is what its creator Toby Fox has called, "a game you can play after you complete UNDERTALE, if you want to,” and it appears to take place in an alternate Undertale universe.
As with Undertale, Fox worked on the game mostly-solo save for his friend Temmie Chang, who did the artwork. However, whereas Fox was able to complete Undertale in a matter of months, Deltarune took him several years due to the more complex nature of the game. Interestingly, Toby Fox actually began working on Deltarune first, back in 2011 and 2012 when he was doing work for Homestuck. When Undertale was funded on Kickstarter, he said he attempted combining elements of Deltarune and Undertale, as well as reusing some of the old Deltarune songs. After the success of Undertale, Fox decided to finish Deltarune after all, and that's what he's been doing for three years.
After Deltarune came out, Toby Fox said on twitter that he was going to bite the bullet and hire on a team to help him, as there was no way he'd be able to do the entire game on his own. However, he also said that he's never directed any kind of team before, and has no idea how long it would take to get the real game completed, so for the foreseeable future, Deltarune Chapter One is all the Deltarune we're going to have.
What's it about?
As previously established, you are Kris. You live with your mother, your brother is at college, your father runs a flower shop, and you are the only human in a town filled with monsters. Why are you the only human? Good question, ask another.
Kris doesn't talk much-- or at all. Nor do they seem to make many facial expressions, and their bangs mostly hide their face. They have some friends at school, including the sweet reindeer monster named Noelle, but unfortunately, they also have to deal with Susie.
Susie is some kind of evil horse-dinosaur monster with fabulous hair and a heart filled with murderous rage. She's the local troubled-kid/school-bully who's one wrong move away from getting expelled, and guess who winds up on her shit-list for seeing her stealing chalk? Of course, since the universe hates Kris, their teacher, Alphys sends Susie out to collect more chalk, and sends Kris to help.
Unfortunately for them, this world runs on Narnia/Alice in Wonderland rules, and the closet is a lot larger on the inside than a closet should be. Then, the floor falls away, and the two land in a strange new world, dressed in new outfits and wielding weapons.
A cloaked figure tells them they are in the Kingdom of Darkness, and that legend has spoken of two Heroes of Light who are meant to join another hero and become Delta Rune Warriors and save the world from an evil angel who seeks to disrupt the balance of light and dark.
Susie, being Susie, is unimpressed and says good luck with that, she's going home.
Before the cloaked figure can convince Susie to stay, they are attacked by Lancer, a lightheartedly malevolent kid on a bike who is trying to impress his dad by thwarting you all. . . His dad being the evil king who has usurped the throne at behest of the greater evil in the land.
Thus begins the proper plot of Deltarune: trying to stop Lancer (who is the harmless, funny kind of antagonist), stop his dad (who is NOT the harmless, funny antagonist), save the kingdom, and stop Susie from killing everyone she meets.
Helping you and Susie -- though mostly you because Susie keeps running off to fight evil, join evil, fight evil, join evil, etc-- is Ralsei, the cloaked figure who removes his cloak to reveal a small, fluffy boy in an enormous hat that shadows his face. Ralsei turns out to be as kind as Susie is mean-spirited, and is the healer of the party. He insists that you do your best not to hurt others and instead use ACT in order to pacify them-- a mechanic many players know from Undertale's pacifist run.
What's different?
There are a few main ways the game differs from Undertale: the characters, the mechanics, and the theme.
The characters are the most straightforward ones: because this is an alternate universe, characters who were previously your friends, or friends with each other, aren't. They retain their personalities, but they have had different lives, so you get a Mettaton who lives in a shut in house instead of being a famous celebrity, or Bratty and Catty, who were inseparable best friends, hating each other's guts. It's simple, but it's jarring to anyone who's spent a lot of time consuming Undertale.
The mechanics are the most noticeable difference. You're not controlling one little person anymore, now you're controlling two or three, and one of them isn't even really under your control. Just like how old school Pokemon games won't let you boss around Pokemon of a higher level, so too does Susie not give a shit about what you want her to do. It becomes a part of the game mechanics that Susie will attack every chance she gets, so you have to work around her, warning your opponents when she's about to swing her ax, or even putting her under status effects like sleep in order to stop her from brutally murdering everyone you meet.
Another battle mechanic change is the addition of TP, "Tension Points", that the characters receive upon getting hit, which is what fuels Ralsei's magic, so battles become a juggling game of getting the enemy monsters to not want to attack you anymore (which enables you to spare them), preventing Susie from killing everyone, and keeping Ralsei juiced enough to heal you all when you need it.
The major thematic shift in the game is that of choice.
Undertale was very intentionally made as a deconstruction of RPGs, and put extensive value on player choice. While much has been said of the Pacifist and Genocide runs, there was also a myriad of medium-endings that varied depending on who you killed and who you spared. In the genocide run, the villain Flowey specifically mocks gamer's need for choice and content, as he himself is living a Groundhog Day existence, reliving the same events over and over just to see what happens if he changes things, going slowly insane and becoming incapable of seeing others as anything but a collection of rote phrases, whereas in the pacifist run, when the player is judged by Sans, the choices are praised as having meaning.
In contrast, Deltarune literally tells you from the start that your choices don't matter. You can't tell Susie your opinions in school; she literally cuts off the choice box before you can say anything and tells you it doesn't matter. You can't pick who your character is, or even name them. They exist already in the context of this new world, and regardless of what you do, every person playing Deltarune is going to get the same ending-- and no, I'm not talking about how you're escorted out of the Kingdom of Darkness. That does have a little variation depending on your actions, but that's not the ending.
It's unlikely that this theme was so upfront in the original drafts of the game, pre-Undertale. It's more likely that the theme was included more strongly now to juxtapose against the previous game and warn players not to expect a hundred different endings based on every meticulous decision they made over the course of the game; Deltarune is expected to be much much longer than Undertale, and while Undertale was a deconstruction of RPGs, Deltarune is definitely a more traditional one.