Hengis Wulfson was a bard and adventurer simply doing what most adventurers do: searching for treasures. On one of his travels to the Plains of Havoc, though, he was imprisoned by Cyclopes in their camp there. In an attempt to occupy their chief to eventually get out alive, he started teaching him to read and write. Unfortunately, he never saw the sunlight again.
In his diary, Hengis makes clear that his last expedition was set out for the treasures of the Nightmare Knights, primarily the Excalibug. He also mentions a song through which he got to know about the treasures and which he now regrets to have so naively sung himself, as he has probably condemned many others to death with it. That song was written down in this book. Hengis's friend Frodo mentions he often sang it while in his tavern, and can recite it for you.
If you are more interested in the progressions his pupil has made under his supervision read this book.