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Comments (3)

What do you think?

TriggerHappy @TrigerHappy

Looks great from the screenshots, currently hoping this thing actually ends up fully made!

GhostDude123 @GhostDude123

I saw this mod concept just a few days ago, I'm really excited for this concept

The world
is as big as you want it to be.
Where will you go next?
Undertale 10th Anniversary Stream Ending Phrase.

REMINDER: Some of the Screenshots are Mockups and not a final product, so expect changes on the future.
REMINDER 2: In order to play that mod you need a legitimate Undertale copy such as on steam.

UNDERTALE: Extended Underground is a fan-made mod that have the purpose to add new areas to UNDERTALE. (which also includes 10th Anniversary ones, And UNDERTALE Yellow ones too.)

Explores these new areas with new monsters, and even some very familiar ones...

*This map can potentialy change over the mod progress.

The mod adds new 14 areas to the Underground.
(6 being from UNDERTALE: Yellow while the other 8 being fan-made.)

Most of the areas lore and info are available, and are still being written on this masterdoc.

The mod gonna be first released to the public as a "Demo" only including Catacombs, and then gonna have Autumnia for the v1.0.0.
Catacombs:

Map Layout: 95%
Story: 5%
Dialogues: 10%
Battles: 0%
Puzzles: 5%
Misc: ???%

Autumnia:

Map Layout: 0%
Story: 0%
Dialogues: 0%
Battles: 0%
Puzzles: 0%
Misc: 0%

Kai-Dusk (Director, Writer, Programmer, Artist and Pixel Artist)
"Berdly" (Pixel Artist)

Thanks for your interest on the mod! :D

UNDERTALE © 2015 by Toby Fox, Temmie Chang

#undertale #fangame #mod #rpg #puzzle

Development Stage

Devlog

Engine/Language

Game Maker: Studio

Published On

May 18, 2026

Mild Cartoon Violence

Mild Fantasy Violence

Alcohol Reference

Mild Language

Comical Shenanigans

Kai.Dusk @Kai-Dusk

Pinned2 days

Some plans for Extended Underground Catacombs Demo (and some plans for v1.0.0 as well), and some other news too.

It sometimes sucks that I need simplify the coloring of some assets, to try fit with Undertale artstyle.

(then vs. now)

A quick explanation of how the Train system gonna work on the Extended Underground mod.

these are the current npcs i've made for catacombs (as of now)

- Gravey (can be battled while on Catacombs.)
- Grimclops (can be battled while on Catacombs.)
- "Slime Widow"
- "Emo Beagle Dog"

I made it to put borders for the mod!

The main issue about this, is that sometimes the borders might not work sometimes.

So the screen keeps on the original size, instead the one with borders.

But it might can works sometimes.

Game Dev

Undertale

UNDERTALE: Extended Underground Mod

Comments (3)

REMINDER: Some of the Screenshots are Mockups and not a final product, so expect changes on the future.
REMINDER 2: In order to play that mod you need a legitimate Undertale copy such as on steam.

Most of the areas lore and info are available, and are still being written on this masterdoc.

The mod gonna be first released to the public as a "Demo" only including Catacombs, and then gonna have Autumnia for the v1.0.0.
Catacombs:

Autumnia:

Recommended

Share

Comments (3)

REMINDER: Some of the Screenshots are Mockups and not a final product, so expect changes on the future.REMINDER 2: In order to play that mod you need a legitimate Undertale copy such as on steam.

Most of the areas lore and info are available, and are still being written on this masterdoc.

The mod gonna be first released to the public as a "Demo" only including Catacombs, and then gonna have Autumnia for the v1.0.0.Catacombs:

Autumnia:

Recommended

REMINDER: Some of the Screenshots are Mockups and not a final product, so expect changes on the future.
REMINDER 2: In order to play that mod you need a legitimate Undertale copy such as on steam.

The mod gonna be first released to the public as a "Demo" only including Catacombs, and then gonna have Autumnia for the v1.0.0.
Catacombs: