Undertale’s 10th Anniversary Stream Just Changed Everything: New Areas and Game-Changing Tweaks Tie It Closer to Deltarune — Fans Are Losing It

Undertale just hit 10, and instead of a quiet victory lap, the official anniversary stream basically poured jet fuel on a decade of fan theories. New footage. Odd little changes. A familiar face (or, you know, lack thereof). And now everyone is arguing about Gaster again. As they should.

So, what did the anniversary stream actually change?

Fangamer and Toby Fox ran a marathon 10th anniversary stream of Undertale, and sharp-eyed fans caught a handful of things that are very much not in the original game:

• An entirely new, moody stretch of ruins that nobody recognized from Undertale proper.
• Gaster's ultra-rare door showed up in full color instead of its usual monochrome look. If you know the door, you know it only appears under super specific conditions — seeing it at all is weird, seeing it in color is weirder.
• Reddit immediately clocked that the newly shown ruins match imagery from Deltarune's intro cutscene.
• Gaster's theme plays in this area, just like it pops up throughout Deltarune, which is only going to fuel the cross-game connection theories.

The fan reaction (and the intentional tease)

The stream did that classic Toby Fox thing where it brushes past the juicy stuff quickly, which only makes everyone fixate harder. Forums lit up, and YouTube comments went feral. One sentiment stood out:

"Toby really knows how to ragebait."

Honestly? Fair. The stream more or less waved a flag that said 'Gaster' and then kept moving. People called it insane, and I get it.

Is this new content actually in Undertale now?

Probably not. As hype as it feels, the new ruins and the colorized Gaster door seem to have been created specifically for the anniversary stream. So if you were hoping for a stealth Undertale patch or DLC drop, temper those expectations.

That said, Deltarune has been heavily teasing Gaster for years, and these ruins lining up with its intro is not exactly subtle. If anything gets expanded, expect it to happen in Deltarune.

Where Deltarune stands

Toby Fox has already tempered the timeline: he does not expect Deltarune Chapter 5 to land in the first half of 2026. The upside? He also said there are no external factors locking down the release date this time, and the team will ship it when it is actually done. Not thrilling for your calendar, but good for the game.

Bottom line: Undertale turned 10 and immediately turned the Gaster speculation dial back to max. No update today, but the Deltarune threads are getting tighter — and we just got a very loud wink about where they might lead.