Written for the LotR Community's 2016 Spooky Atmosphere challenge, the prompt being Gollum's third riddle.

It cannot be seen, cannot be felt,
Cannot be heard, cannot be smelt.
It lies behind stars and under hills,
And empty holes it fills.
It comes first and follows after,
Ends life, kills laughter.

Pippin wished fervently that he could stop running over in his mind the riddle he had heard old Bilbo repeat time and time again as he recounted the tale of his encounter with Gollum so many years ago.  Bad enough it was to be thinking of Gollum when Pippin was surrounded by darkness on all sides with not even a spark to relieve the heavy blackness pressing in on him.  For all he knew, that wretched creature was out there somewhere, waiting to attack or to grab him from behind when his guard was down.  Far, far worse was the thought of empty holes filled with darkness.  He had just such an empty hole yawning wide and dark behind him, as he sat by the door of the chamber, on watch while the others slept, and he could not stop thinking about it.

What if something crawled up out of the hole?  Would he hear it in time to warn the others?  Or would that thing creep silently out of the old well, to catch him from behind before he even knew it was there -- grabbing him with sharp claws, perhaps dragging him away, ending life, killing laughter?

Pippin swung around and stared in the direction of the hole, but nothing was visible in the pitch dark.  He strained his ears to listen for the slightest sound -- of the movement of stones as something crawled up out of the depths, of breath whistling through sharp teeth, or of something slithering closer and closer out of the darkness -- but there was only the quiet sound of his companions' even breathing as they slept.

He turned back, and tried to think of something less frightening, but it was no use.  Fear made him shiver as the pit behind him poured out darkness so that he could almost feel it approaching in order to engulf him.  If only he could cover it with something, to keep the darkness inside!  He hunched his shoulders, struggling to not look, to not turn around.  If something was coming, he did not want to see it...

A hand on his shoulder made his heart leap in his chest and he jumped in fear -- but it was only Gandalf.  "Get into a corner and have a sleep, my lad," he said kindly.  "I'll do the watching."

Pippin complied with relief.  Gandalf would make certain no evil creature crawled up out of that hole, he would be safe now.  As he settled himself in his corner, a spark of light flared up as Gandalf lit his pipe.  The darkness receded, and Pippin saw that all was as it should be.  The mouth of the well was just a hole in the floor, nothing more.  Nothing crawled up out of it, for there was nothing there.  He was safe and his friends were safe, sleeping within reach if he needed them, and Gandalf was on guard.

He closed his eyes and the darkness that came as sleep took him was welcome.