Anselm was a Fallen Knight who died shortly before the events of Hearts of Stone Expansion for The Witcher 3: Wild Hunt. His orders from Grand Master Ulrich were that he traveled to Novigrad to resupply and sell latest batch of fisstech. Customers however bertrayed the deal and instead killed knights under Anselm's command leaving the knight sole survivor. They would also kill him, but they needed key to chest with fisstech. It turned out that they can not find mentioned key because knight slipped it into his arse.
Even though his captors were later killed by drowners, he could not escape because they locked him up in the cellar of hut, place of the meeting. After tree days he had nothing left, nor food, drink, not even hope so he decided to write a letter and than die.