Undyne the Undying is one of the most demanding boss fights in Undertale — the terrifying evolved form of the Royal Guard captain that appears exclusively on the Genocide route when players attempt to kill every monster in Waterfall. She is widely considered one of the hardest fights in the game and has spawned a dedicated community of fan-made fight simulators that let players practice her attack patterns, experience the fight without completing the full genocide run, or just test themselves against one of gaming’s most memorable superbosses.
This guide covers the best Undyne the Undying fight simulators available online and on GitHub, how to play them, what makes the fight so difficult, and tips for surviving her legendary attack sequences.
Who Is Undyne the Undying? (Undertale Context)
Undyne is the captain of the Royal Guard in Undertale — a passionate, determined fish monster who serves King Asgore and is one of the game’s central characters. In the Pacifist and Neutral routes, she is a complex antagonist who ultimately becomes an ally. In the Genocide route — where players kill every monster in each area — her character arc changes dramatically.
When the player kills every monster in Waterfall and attempts to proceed, Undyne refuses to die. Despite being mortally wounded, she transforms into Undyne the Undying — a powered-up form driven by sheer determination to protect all remaining life from the player’s destruction. She screams ‘Human. I’ll be the one to erase your SOUL!’ and the hardest standard boss fight in Undertale begins.
Undyne the Undying was designed by Toby Fox and character artist Temmie Chang. She has appeared in Undertale (2015) and, in a non-combat role, in Deltarune Chapter 3 (2025). Her battle theme ‘Battle Against a True Hero’ is one of the most beloved tracks in Undertale’s soundtrack.
What Is the Undyne the Undying Fight Simulator?
The Undyne the Undying fight simulator is a fan-made recreation of her Genocide route boss fight — typically built in a browser (JavaScript/HTML5) or as a downloadable game using engines like GameMaker, Python with Pygame, or the Create Your Frisk (CYF) Undertale fan game engine. These simulators let players:
- Practice her attack patterns without replaying the full genocide route
- Experience the fight if they have not played (or do not want to play) the full Genocide route
- Challenge themselves with fan-made harder or modified versions of the fight
- Play on platforms where the original Undertale is not available
The simulator community grew from the same ecosystem that produced the Bad Time Simulator (the Sans boss fight replica) — arguably the most famous Undertale fan game ever made — and follows the same pattern of fan developers recreating specific fights as standalone browser experiences.
Best Undyne the Undying Simulators to Play Right Now
1. Fairdyne / NGAHHH! Simulator (Online — Free)
URL: https://sans-simulator.github.io/undyne-simulator/
The most widely played browser-based Undyne the Undying simulator is Fairdyne (also called NGAHHH! Simulator v0.99), created by Joe Zeng and hosted on the same GitHub Pages domain as the popular Bad Time Simulator. It is free to play in any modern web browser with no download required, making it the go-to recommendation for anyone looking for an instant online version.
- Controls: WASD or arrow keys to move the soul; Spacebar or Enter for Determination (temporary invincibility) / skip text
- UNBREAKABLE MODE: Accessible from the pause menu — activates a fun musical event while removing the extreme difficulty for players who just want to experience the fight’s atmosphere
- Source code: Available on GitHub (linked from the game page)
- Disclaimer: Fan-made project; Undertale is the property of Toby Fox
2. Undyne-Simulator by darshg321 (GitHub — Python/Pygame)
GitHub: https://github.com/darshg321/Undyne-Simulator
A Python and Pygame implementation of the Undyne boss fight with random attacks. This is an open-source downloadable project for players who prefer a local application over a browser game, and for developers interested in the source code implementation. Requires Python and Pygame to run.
- Platform: Windows/Mac/Linux (any system with Python installed)
- Source code: Fully open-source on GitHub — good reference for developers
3. Create Your Frisk (CYF) Custom Battles
Create Your Frisk (CYF) is a fan-made Undertale battle engine that allows players to run custom boss fight scripts. Multiple community-created Undyne the Undying recreations and harder versions exist as CYF mods. The standard version can be found on the CYF GitHub, and Undyne the Undying battles are available as downloadable mod files that go in the Mods folder.
- Requirement: Download CYF 0.6.5+ and place the battle in the Mods folder
- Best for: Players who want more authentic recreation or harder modified versions of the fight
- Community: Active Undertale modding community on Reddit (/r/Undertale) and Discord
4. Scratch-Based Undyne Simulators
Multiple Undyne the Undying fight recreations exist on Scratch (scratch.mit.edu) — the block-based programming platform popular with younger developers. Search ‘Undyne the Undying’ on Scratch to find community projects. Quality varies significantly between projects but some well-built ones capture the core fight mechanics. These are playable directly in the browser through the Scratch website.
- Platform: Browser (no download required)
- Quality: Varies — look for projects with high view counts and positive ratings
| Simulator | Platform | Download? | Difficulty | Best For |
| Fairdyne / NGAHHH! Simulator | Browser (online) | No | Hard (Unbreakable mode available) | Quick play; most authentic browser version |
| darshg321 Undyne-Simulator | Python/Pygame | Yes | Variable (random attacks) | Developers; local play |
| CYF Custom Battles | Windows app | Yes | Hard to Very Hard | Authentic recreation; modded versions |
| Scratch Projects | Browser (online) | No | Variable | Casual; beginner-friendly versions |
How to Play the Undyne the Undying Simulator
Basic Controls
- Movement: Arrow keys or WASD — moves your soul (the heart) around the battle box
- Spacebar / Enter: Determination (in some simulators) — activates temporary invincibility for a short duration, used strategically to survive unavoidable hits
- Z / Enter: Confirm selections during action menus
- X / Shift: Cancel / back in menus
Understanding the Green Soul Mechanic
Undyne the Undying is the only standard boss in Undertale that turns your soul green. The green soul mechanic flips the gameplay from evasion to blocking — instead of moving your soul to avoid attacks, you must face the direction of incoming spears and press the correct arrow key at the right moment to deflect them with a green shield.
This is fundamentally different from every other Undertale fight and is the primary reason the fight is so challenging — players must switch between the standard red soul evasion and the green soul blocking mode at Undyne’s command, often rapidly.
Undyne the Undying Attack Patterns
Undyne the Undying has a large number of attacks that escalate in speed and complexity as the fight progresses. Understanding each pattern is the key to surviving.
- Spear walls: A row of spears launches from one side of the battle box — move to the gap or deflect with the green soul depending on which soul color is active
- Spear rain: Multiple spears target your position from above — move constantly to avoid or deflect in rapid succession
- Green soul blocking phases: Undyne commands you into green soul mode — incoming spears must be deflected by pressing the correct directional key when the spear arrives
- Speed escalation: The fight gets progressively faster as Undyne’s HP decreases — attacks that were manageable at the start become extremely rapid near the end
- ‘Run it!’: Undyne periodically commands you to run — moving your soul to the edge of the battle box during these moments
Tips to Survive Undyne the Undying
- Master the soul color switch: The single most important skill is quickly recognizing when your soul turns green and immediately switching from evasion to deflection mode. Green soul = block, red soul = dodge. This sounds simple but the transition happens rapidly under pressure.
- Stay near the center early: In the first phases, staying near the center of the battle box gives you maximum range to move in any direction when attacks come. Moving to a corner early limits your options.
- Anticipate spear direction: Most spear attacks target where your soul currently is, not where it will be. Move as the attack launches, not as it arrives — by the time a spear reaches you, it is too late to react.
- Use Determination wisely: In simulators that include the Determination mechanic (temporary invincibility), save it for the densest attack patterns rather than using it immediately when you get hit.
- Practice in segments: Use the simulator to practice specific attack patterns that kill you repeatedly. The early phases are more forgiving — focus on mastering the mid and late fight patterns where most players fail.
- Use Unbreakable Mode first: If you are new to the fight, activate Unbreakable Mode (available in Fairdyne) to watch the full fight, understand the attack sequences, and learn the patterns before attempting the real difficulty.
- Calm your inputs: Undyne the Undying causes panic that leads to over-correcting movements. Small, deliberate movements survive more attacks than panicked large movements across the battle box.
Is the Undyne the Undying Simulator Available on Mobile and Android?
The browser-based simulators (Fairdyne / NGAHHH! Simulator) can be accessed on mobile browsers, though the experience is significantly diminished without physical keyboard controls. The fight requires precise directional input that is difficult to replicate accurately on a touchscreen.
There is no official dedicated mobile app for the Undyne the Undying simulator. For the best experience on Android, consider using a Bluetooth keyboard with a browser for the online version, or look for a Scratch mobile project that has been optimized for touch controls. The original Undertale is available for Android and iOS, which includes the actual Undyne the Undying boss fight in its full form.
Undyne the Undying vs Bad Time Simulator: Which Is Harder?
The Bad Time Simulator (the fan-made Sans boss fight recreation) is more widely played and more frequently cited as the hardest fight in Undertale. However, the difficulty comparison between Sans and Undyne the Undying is a genuine point of debate in the Undertale community.
| Sans (Bad Time Simulator) | Undyne the Undying | |
| Fight mechanic | Red soul evasion throughout; complex multi-phase patterns | Switches between red soul evasion and green soul blocking |
| Why it’s hard | Extremely fast, high damage, bone patterns require memorization | Green soul mechanic is unique; escalating speed; long fight |
| Community consensus | Generally considered the hardest standard fight | Second hardest; simpler mechanics but still very demanding |
| Simulator availability | Bad Time Simulator (Jcw87) — very polished | Multiple simulators; Fairdyne most popular |
| Attack variety | Very high; multi-phase attack rotation | High; green/red soul switching adds complexity |
Most players who have beaten both agree that Sans is harder overall, primarily because of his extreme speed and the bone-gaster blaster combo patterns in his final phase. Undyne the Undying is considered the harder fight for players who struggle specifically with the green soul mechanic, which has no equivalent elsewhere in the game.
Frequently Asked Questions
Is there an Undyne the Undying fight simulator I can play online for free?
Yes — Fairdyne (NGAHHH! Simulator v0.99) at sans-simulator.github.io/undyne-simulator/ is free to play in any modern browser with no download. It includes an Unbreakable Mode for players who want to experience the fight without the full difficulty.
Where can I find the Undyne the Undying fight simulator on GitHub?
The darshg321/Undyne-Simulator repository on GitHub (github.com/darshg321/Undyne-Simulator) is a Python/Pygame implementation of the fight. The source code for Fairdyne is also linked from the game page. Additional projects can be found by searching ‘undertale undyne’ on GitHub’s topic search.
What is the Determination mechanic in Undyne simulators?
In some simulators, the Spacebar activates Determination — a reference to the game mechanic of Undertale where the player character survives through sheer determination. In simulator context, it typically provides a brief window of invincibility to get through unavoidable attack overlaps. Not all simulators implement this feature.
Do I need to play the full Genocide route to fight Undyne the Undying in Undertale?
Yes — in the original Undertale, Undyne the Undying only appears if you have killed every monster in Waterfall before confronting her. This requires committing to the full Genocide route up to that point. The fan-made simulators exist specifically so players can experience the fight without completing the genocide run, which many players find too emotionally difficult to complete.
Is Undyne the Undying harder than Sans?
Most players and community consensus rate Sans as the harder fight overall, though Undyne the Undying is genuinely difficult and specifically challenging for players who struggle with the green soul blocking mechanic. Undyne the Undying is a longer fight with a unique mechanic switch; Sans is faster and more punishing in his final phase.
Final Thoughts
Undyne the Undying is one of the most memorable boss fights in gaming — not just for its difficulty but for the emotional weight behind it. She is not fighting because she is evil; she is fighting to protect everyone she loves from a player who has chosen to destroy them all. The fan-made simulators let players experience and practice that fight without the full emotional and gameplay commitment of the Genocide route, and the most polished of them — Fairdyne — does a genuine job of capturing what makes the original so intense.
If you are practicing to beat the actual fight in Undertale, the simulator is an excellent training tool. Master the green soul timing, stay calm during the speed escalation, and remember that every Undertale boss fight — however many attempts it takes — eventually clicks.
All simulators mentioned are fan-made projects. Undertale is the original work of Toby Fox.
