Accessing the Stairway to Heaven

Ein stared at the key, admiring its unique design. It was made of what looked like bones, especially a human skull at the bow or handle. The teeth along its sides were literal teeth, whose sizes and sources varied greatly.

[That was quick. Did you figure out that reading it twice is the answer?]

With Nyarlathotep's casual comment, Ein realized what sort of trick Yog-Sothoth had played. Reading the letter for the first time causes one to have extremely unpleasant feelings, enough to make one lose their mind. After experiencing that even once, nobody would even think of reading it again.

In short, the test was to see if anyone had the willpower and was brave enough to try and face the same unpleasant things they experienced once. It was a pretty simple test, but a rather nasty one.

"... I really hate Yog-Sothoth's tastes in these sorts of things."