Derril is a character mentioned only on page 21 of my notes, a collection of pages recovered by the player during and after the Barbarian Training miniquest. He does not appear in-game, although some details regarding his history can be gleaned from the notes.
According to the page, Derril was involved with a group of explorers that entered the Ancient Cavern beneath Baxtorian Falls at an undetermined point in the past. The notes suggest that the group attempted to open the mithril door found near the cave's centre through a variety of methods, including the fishing explosives normally utilised in the slaying skill to kill mogres. Evidently Derril was the one that suggested the group use the explosives, although whether he was part of the expedition or merely offered them advice prior to their entry is unknown.