Merchant Encounter

At a bend of the forest road, your party stumbles upon the overturned cart.

It looks old; the wood is rotting, but the road is clearly well traveled. It lies there almost too perfectly, and no one is around. After some time, the adventurers hear overlapping voices, as if a merry fellowship is passing by. The voices quickly turn into tense whispers until they hear a sharp scream. Then, one of the adventurers sees the cart with people crashing at the same spot. The party hears the voices again, and then they all see the crash. Only, the cart is somewhat transparent, and the people in it seem unaware of the adventurers. As if the event keeps replaying itself. What is going on?