Nil

What is the Citadel of Abolished Vacuous Desolate Abyss?
nil is a humanly impossible difficulty, used to be the second hardest difficulty in SqTs.

As it is above tier 17 and so, it has the sign, "BASICALLY IMPOSSIBLE".

List of Towers with nil Difficulty

 * 1) Citadel of No

Trivia

 * nil is the SECOND HARDEST difficulty in SqTs