There are several riddles in the game, most given by Imrahis in Bavelock. He will tell you a random riddle out of 8 from the riddle book he is reading when you talk to him.

In the main story of the game, just before you enter the Thieves' Passage the ghost Agga asks you a riddle that you must answer before you can pass.

"Soundless it flies. It's faster than me. In darkness it dies. It's all we can see."
-Light. (Answer in older versions - V 2.5 or lower - was 'Fire')

In the "Buying ink" sidequest, Padamus gives you a riddle.

"Life it devours, forest, grass or flowers. But it can not be, without human hands. It's useful you see, but help it demands."
-Fire. (Answer in older versions - V 2.5 or lower - was 'Light')

Here are all of Imrahis' riddles and their respective answers:

"At night it shows how bright it glows. At day it hides above the skies. Two answers at stake, which one will you take?"
-Star or moon.

"It grows and grows the more you take. But nothing it shows. It's nothing you make."
-Hole.

"Green and tall. Big and small. Armless it's not. They gather a lot."
-Tree.

"Bigger than earth. At night not found. Unseen at birth. High above ground."
-Sun.

"Holes in a row. In mud or snow. The more you are, twice more they grow."
-Footsteps.

"The maker makes it but doesn't use it. The buyer buys it but doesn't need it. The one who needs it never knows it."
-Coffin.

"One it is, fits on ten. In silver or gold. Worn by many, but icy cold."
-Ring,

"Gives us life, takes our stress. Yet makes us weak, and makes us helpless."
-Sleep.