I don’t actually know physics, but I found a realtime black hole shader written by ebruneton and ported that to Godot. The original author even has an paper about the shader that you can read on Arvix!
Unfortunately, the gdscript/gdshader code is kind of mess and I’m a little unhappy with how some of it works. I had to skip over some parts that I didn’t really understand or have time to implement. After I’m done making the game, I’m hoping I’ll have some time to clean it up a bit and make the ported shader available on godotshaders.com and GitHub.
Well, to be fair, time literally doesn’t flow the same as it would on Earth around a black hole due to… uh… gravity warping the space-time continuum. Or something like that.