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He HOPED you wouldn't show up.
There must be a reason you did...

Features custom artwork, attacks, and music, and 3 endings! (Plus a secret one...?)
Also try the original Thundertale!

Click inside the game to get your keyboard controls to work!

[Arrows or WASD] - Move
[Z] or [Enter] - Confirm 
[X] or [Shift] - Cancel

[-/+] - BGM Volume
[;/'] - SFX Volume
[ESC] (Hold, while in combat) - Reset
[321] - Graphics quality

If you'd like to download, also download index.html and put it in the same folder as the SWF. Then you can open index.html in a browser and the game will play (and scale up in size).

You'll probably need a standalone Flash player.

Flash port, art, and battle by Serule Blue.  Undertale and SFX by Toby Fox.  BGM by Checqz.

Updated 16 days ago
Rated 4.0 out of 5 stars
(2 total ratings)
TagsAction RPG, Bullet Hell, flash, Pixel Art, Singleplayer, Undertale
Asset licenseCreative Commons Attribution_NonCommercial v4.0 International
Average sessionAbout a half-hour


ThundertaleGX.swf 7 MB
index.html 246 bytes

Development log


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And thus, the deed is done. I thank you, Byte. For trying to keep creatures such as myself at bay, so that others may live. I thank you Blue, for believing in me no matter what. And finally, I thank you, Serule, for an experience that gave me so much joy, and fun. For showing me characters that gave me a challenge, and a laugh. Thank you. I will support you on your future adventures, and wish you the best of luck.

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Congratulations! And thanks for playing!

is there a way to plat after flash died?

You can download the .swf file and a standalone Flash player. Then just use the player to open the .swf.

i like the game and i hope you do more

The secret ending is dope!

Also question for Serule. What are the two downloads for seeing as this is a flash game?? Genuinely Curious Here.

Ummmmm Sooooo There's A glitch with the game if you go into the secret and reset during the break point then go into a regular game it wont let you fight.

I absolutly adore this game just beat the secret ending IT WAS HARDER THAN SANS love it! I hope you make more projects!!!


Xd took me 50 tryes to beat this um this game is not for new undertale playes


plz make more

(1 edit) (+1)

This is absolutely amazing. I have no words.

Also, <ESC> doesn't seem to work for me

Thanks! <3

And are you holding it down? Just tapping it won't work (so you don't accidentally reset). It also only works during the battle, not in any of the menus.

OMG THE SOUNDTRACK IS INCREDIBLE AND THE SPRITES AND ATTACKS AND JUST IT'S A WORK OF ART! I played it 45 min after it came out and it is amazing

Thanks so much! I'm glad you like the new animations; I didn't originally plan on making them, but eventually I decided that Blue just standing there wasn't exciting enough :)

And thanks on the music, too. The game's musician appreciates your feedback!

Really cool! So glad you decided to make more of these, I really liked the first one. Will you add WASD support? My up arrow key is busted lol. Also what will happen after Flash gets shut down?

Also can you add a graphics settings? The battle looks really cool and all but it lags and makes it hard to dodge stuff

(1 edit) (+1)

Thanks, glad you like! And sure, I can add both of those in an update. Thanks for the suggestions!

Also, this will be the last Flash game I make. But if you'd like to play these after Flash is killed, you can still download the standalone player from Adobe's website. I'll add a link to it once it's around that time.


I just uploaded v1.01. It adds [WASD] and quality controls [321]. I hope that helps!

Thank you so much! Sorry for the late reply, I'm not on itch.io that often.