Marshall Tasca is a renowned explorer out of Ascalon. Together with his good friend Marshall Whitman, they set out to explore the known world about a dozen years prior to the events in Guild Wars Prophecies. They are famous for their thorough exploration of the Maguuma Jungle and the Shiverpeak Mountains. The Deldrimor War Camp was initially established by them as their base camp for the mountain expeditions.
About a year prior to the events in Prophecies, Tasca fell off a cliff while exploring in the mountains. Whitman has been searching for him since. The exact location where Tasca fell is unknown, although the explorable area Tasca's Demise would appear to have been named to commemorate the famous explorer. Interestingly, there is also another area known as Witman's Folly. Whether it refers to Marshall Whitman is unknown.
In Prophecies, players may chance upon Marshall Whitman, who would ask the players to help him find proof of Tasca's death or survival in the quest Tasca's Reprise.