How to get the 'dirty hacker' ending in Undertale
Undertale has a lot of endings, and creator Toby Fox has accounted for every possibility in getting them. There are the True Pacifist and Genocide endings at both ends. The neutral paths have at least ten unique endings (and counting?). Finally, there's the "dirty hacker" ending, also known as the "impossible ending" because, well, it's impossible to get.
Unless you're a dirty hacker, of course!
In order to achieve the ending, you have to access the files that Toby Fox used to create the game. These are not attainable via the local files downloaded with the executable game file. But, you have to find a way to access the code in the game itself.
If it's difficult to see in the video, Youtuber Mir Rawrs is using XVI32, a hex editing program to access the game's binary code by presenting it in a hexadecimal pattern. In the video, the creator swaps the code for the ending he is about to get with the impossible ending.
The byte changed in the file is the value assigned to the variable that determines the ending, given that the conditions to trigger it are satisfied. The two values edited in the save file are the conditions to make the former event occur. In other words, I'm replacing an ending with the one in the video.
- Mir Rawrs explains their process in the Youtube comments
Theoretically, you can also get this ending by setting two contradictory values, like that you did and did not kill a certain character. However, you would have to find a way to do that without crashing the game.
Toby Fox seems to be the master at tying up loose ends. The fact that he anticipated hackers and addressed them directly make for a fun easter egg, to say the least.