Return to the survival horror of the 90s - terrible controls and all!
If you're a fan of classic Resident Evil games or looking to torture yourself, you might enjoy this small taste of panic and frustration! While you scavenge rooms for keys and weaponry, a demon lord works behind the scenes to destroy rooms and leave you stranded with no escape from this nightmare.
https://ldjam.com/events/ludum-dare/54/ld-54
In the meantime, we're playing your games - did anyone else make a survival horror-esque game?
We'll just load ours.
Once you're done with that, throw a sphere in the middle. This will be our black hole.
For this tutorial, we'll make a visual shader, but you should be able to easily replicate this in code. Visual shaders are just a bit more accessible to everyone. So create a new shader material, create a visual shader for it, and set it to VisualShader.
Once that's all done, set shader flag Unshaded to true
We'll now proceed in the visual editor.
This sequence of nodes will apply the flip with heavy distortion along the edges.
We now need to smooth out the edges to make it less jarring. 
You can see this by hooking it straight to the albedo output.
Changing the power values in the fresnel and the floatop will change the look of the fade, experiment later to see what works best for you. For our game, we hooked the fresnel output to a shader parameter.
Adding a multiply before adding to the screen UV lets us gently ease into the flipped look. 
if we multiply the fresnel, we can adjust the size of the black hole. and by rounding, we'll send it to either 0 or 1. Again, we can preview by hooking it up to the albedo.
Increasing the multiplication factor (which we've hooked up to a FloatParameter) will decrease the proportional size of the black hole.
Finally, if we multiply the end results of our two branches together, we'll have a black hole!
But watch out! Due to how the shader works, there are some limitations.
For our game, we added a script which would detect if the camera was inside the black hole or not, and swap out the shaders. (Due to how Godot works, it will recompile the shaders on run time, you'll notice the first time you get close to the black hole in our game, it will lag briefly)
The solution is to change the draw order of things to have the black hole last, but the fastest way we found was just to apply alpha scissor to our transparent objects and deal with the sad looking edges.
It's possible to fix this by being clever with the draw order, but in our game, we just made sure the camera was never in an angle that would let you see any of these glitches.