Nil

nil, originally known as DEATH, is a removed difficulty in JToH that is represented with the color gray. This difficulty was the hardest difficulty that existed, aside from 
TooHard, which is generally not considered a difficulty. nil used to be called "DEATH", as it seemed (and still seems) humanly impossible. Jumps of this difficulty are insanely accurate, require difficult glitches, or are just plain stupid. One extremely tiny mistake can cost you lots of progress or even your entire run, and there is a bigger element of luck involved, because small physics mistakes may screw you over very easily. 
As of May 17, 2020, this difficulty has been removed from the game as a whole, due to it being unneeded with the cancellation of Citadel of Void. Towers that enter the whitelist as nil will not have a real difficulty, which might be nerfed back down to 
Catastrophic. Tower of Slight Inconvenience was moved to this difficulty for a short amount of time as a joke due to the difficulty being inaccurate, but was quickly bumped down to 
Hard. Tower of Spiralling Heights also used to be here, but was bumped down to 
Easy. CoV had this difficulty, but a few days before the Tower Rush was released, it was taken out of the game completely.