Berlong is the twin brother of Leblong the dragon, who was most likely, though not certainly, dispatched by the warlock Grawl. Grawl had agreed to dipose of Berlong in exchange for Leblong's service in guarding a cavern. While Leblong never witnessed the aledged death of Berlong, Grawl at least reported to Leblong that he had done so.

SOURCE(S): Zork: The Cavern of Doom