This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down.J.R.R. Tolkien (1892-1973) English writer, fabulist, philologist, academic [John Ronald Reuel Tolkien]
The Hobbit, ch. 5 “Riddles in the Dark” [Gollum] (1937)
(Source)
One of Gollum's riddles for Bilbo. The answer is "time."