This mystery can be found in the southeasternmost point of Snotinghamscire, to the west of Wenloch Abbey.
An Efficient Cremation: How to Burn the Cart
As you approach the area, you’ll notice a man weeping and begging for help. Once you’re near enough for him to notice, he’ll begin to tell you that he needs to burn his family before they return as vengeful draugrs. To do this, you’ll need to take a fire pot from nearby (you’ll find one behind some regular pots to the north). Throw the fire pot on the cart, or place it nearby and shoot it to ignite it. Either way, once the pot has exploded near the cart, the man will thank you.
You can now speak with the man and learn more about the situation, revealing that he killed his family. He will ask that you kill him and send him to Helheim. When the man is dead, the mystery will complete.