Dieter was a former stable hand of Lord Ignatius Verrieres and served alongside Mislav at the castle.
He discovered Mislav's secret relationship with the lord's son, Florian, after walking in on the couple in the stables, and outed them to the lord. This led to Mislav being banished from the castle and village, Florian hanging himself, and the grief-stricken lord letting the estate fall into ruin.
In May of 1272, Dieter was killed by wild dogs and his body was discovered by Geralt of Rivia and Mislav while the witcher investigated the griffin attacks in White Orchard.