The Forgotten Library

From the introduction:

This adventure can be dropped on the adventurers at any point in time when they are at level 3. The hook is straightforward.

As the party walks along the road, a letter materializes at the feet of one of the players. This letter can contain any number of backstory motivations. Perhaps it details the incarceration of a loved one, or perhaps the location of a powerful item someone is looking for.

Upon opening the letter, as the character reads it, the world starts to shift around the party, as the letters in the letter start making less and less sense. The adventurers find themselves on a snowy road.