Give me your hardest riddles

My party is freakishly good at riddles. They love the riddles, but  I need to actually give them a challenge. What is your biggest, hardest, scariest riddle? 

I've shared this a thousand times, but my most successful riddle was the non-riddle riddle. You have a door with a Magic Mouth spell cast on it. Make the door as elaborate or simple as you want, whatever fits your campaign. Could also be a treasure chest, whatever. When the players approach the door, the mouth triggers and delivers a "riddle." The one I used for my campaign was an adaptation of the Sphinx riddle, something like "What walks on its feet in the morning, Swims during the day, but flies at night?" It doesn't matter what the riddle is, the only thing that matters is that it sounds plausibly solvable, and has no answer. The players will drive themselves crazy trying to solve it - my players blew through a two-hour session doing anything they could to find a solution, even after I let them scour the internet for answers.

The trick? The door is unlocked. No traps. No obstacle of any kind preventing them from just pushing the door open and walking through.

EDIT to add: If you want to add an extra layer of frustration, have the mouth repeat the riddle every time they make a guess.

I have forests with no trees, rivers with no water, cities with no people... what am I?

A. A map

There's this really fun riddle from Tales from the Yawning Portal, and it's in the 2014 Monster Manual. I've always liked it. I don't actually know how difficult it might be for your party, but it's worth mentioning here:

Round she is, yet flat as a board

Altar of the lupine lords

Jewel on black velvet, pearl in the sea

Unchanged yet ever changing, eternally

It's the moon

This isn't really a riddle that has any actual solution, but I based a one-shot campaign around it and created a solution. The story was that an ancient wizard named Kershawnik had created a library that had been lost ages ago and is rumoured to be priceless. Many had tried to find it throughout the ages. The only clue into finding the library was this gibberish a person was saying over and over again that the party met "at random":

Near a tree by a river there's a hole in the ground,
where an old man of iron goes around and around.
And his mind is a beacon in the veil of the night,
for a strange kind of fashion there's a wrong and right.

This is of course (most of) the chorus of a song called The Riddle by Nik Kershaw with slightly modified lyrics (Aran --> Iron). The solution was that they had to dig a hole near a tree by a river. They found a small ancient iron statue that had a joint in its waist. They had to rotate/wind the torso, so that when they let go it unwound and the head started glowing. They had to place the glowing statue into the hole with any piece of clothing at night, so that the entrance to the library was revealed.