It's called the challenge. I won't spoil how you find it, but it's extremely well hidden and even just getting there can be a pain without looking up a guide. Once you get there you have something like 3 minutes to finish a variety of puzzles, each of which is randomly generated (one of which is randomly generated based on your solution to a previous randomly generated puzzle). They aren't insanely difficult per se, but you definitely need pretty good mastery of all the mechanics to get them done.

Unfortunately, and sorry for the spoiler, but the reward is disappointing. Mostly worth it for the steam achievement. If you like philosophy videos then it unlocks one of those. If you don't know about the philosophy videos then there's a lot about the game you haven't found yet.