How to Fight Pacifist Sans and All New Undertale Content From the 10th Anniversary Stream

Dimas Ibnu — Mon, 22 Sep 2025 22:29:42 +0000

Undertale just turned ten, and Toby Fox isn’t letting the milestone pass quietly. To celebrate the anniversary, a special two-day livestream was held, drawing in hundreds of thousands of fans around the world. Alongside the broadcast came a brand-new browser game on the official Undertale website, letting players take on none other than Sans in a fresh pacifist-style fight.

Image Credit: Toby Fox

The Pacifist Sans Battle

The new mode is now live at undertale.com/10 and it flips the usual expectations of a boss fight. Instead of attacking, your goal is to survive as Sans tries to push you off the edge of the battlefield. It’s all about quick reflexes, careful movement, and perfectly timed jumps.

Trying to fight back won’t get you far, since Sans dodges every attempt. Worse still, failed attacks can trigger a brutal bulldozer move that shoves you closer to defeat. The only way to win is to endure long enough without being forced offscreen. It’s a clever twist that plays into Undertale’s themes of choice and pacifism.

The browser game also includes the chance to face Mr. Sunshine and Aberrant, the quirky duo that made their debut in the anniversary livestream.

A Celebration Packed With Surprises

The anniversary stream wasn’t just about Sans. Fans were treated to an exclusive look at new areas, dialogue, and lore snippets that expand the Underground in fun ways. Highlights included:

A new cradle in Toriel’s house
An expanded Ruins section and Snowdin Forest area
A mysterious elevator and new NPCs in Snowed Inn
The Wrong Number Song making a return
Verse Sans and Cook Dude Papyrus moments
A cameo from Deltarune’s Noelle Holiday
Spamton crashing the party
The return of the Gaster Door
A brand-new underwater area
New Hotlands houses, plus Hotlands High School
A fresh Papyrus and Undyne scene
Extended Mettaton content, including an anime confession
New art, dialogue, and even a touching Sans hug
Gerson appearing in the old ruins

These scenes won’t be released as a standalone game, but Fox explained during the stream that they serve more as a “what if?” exploration of the Underground. The idea is to remind fans that the world of Undertale is as big as your imagination, not just what’s officially in the code.

Music, Memories, and Charity

The event also featured exclusive music tracks and dialogue, all now available to download from the Undertale website. Fans also shared heartfelt stories about how the game impacted them over the years, creating a real sense of community around the celebration.

On top of that, the anniversary helped raise over $300,000 USD for Doctors Without Borders, giving the occasion an even greater impact.

Watch the Anniversary Streams

For those who missed the livestream, both days can be rewatched in full. We’ve included them below so you can experience every reveal, joke, and surprise for yourself.

Day 1 Stream:

Day 2 Stream:

Where to Play the New Battles

You can jump into the Pacifist Sans fight and other browser battles right now at undertale.com/10. It’s free to play and a perfect way to revisit one of gaming’s most beloved indie worlds ten years on.