The First Dungeon Master was the descendant of a servant from the court of Lord Dimwit Flathead who was present on the day that Megaboz cursed the kingdom. The FDM inherited the parchment from him as a family heirloom.
In an attempt to forestall the Curse, found the relics of the Twelve Flatheads, met Barbazzo Fernap, travelled to Fenshire and Fublio Valley and also walked on the Great Brogmoid. During his travels he accidentally released grues who thence roamed the lands. His finding of the Jewel of Jerrimore however fulfilled the Curse.
Eventually he came face to face with Megaboz himself. Despite his efforts, the Curse was fulfilled and the Empire was destroyed. However, for reasons that are not entirely understood, Megaboz felt it necessary that someone be left as a guardian or inheritor of the vast underground territory. Megaboz thus gave the adventurer total control over the Eastland caverns, powerful magical abilities, and half the wealth of the kingdom.
As the years passed and the underground caverns became the stuff of legend, the existence of the Dungeon Master was forgotten. In 948 GUE a brave adventurer dared to enter the caverns near the former site of Flatheadia and found his way to the home of the Dungeon Master himself, who, unbeknownst to the adventurer, had been following him and guiding him on his quest.
Apparently the first Dungeon Master grew old and weary of his powers, and had chosen this young adventurer as his successor.