I'd like to point your attention to a neat old public domain book (more properly "books") I came across while scouring through Archive.org entitled Popular tales and romances of the northern nations. It's a 3-volume set of stories collected from Germany, translated and published in England in 1823 (links to each volume via the column headers below).
The Enchanted Castle
Wake not the Dead
The Spectre Barber
The Magic Dollar
The Collier's Family
The Victim of Priestcraft
The Field of Terror
The Fatal Marksman
The Hoard of the Nibelungen
The Erl-King's Daughter
Volume I starts off with a story titled "The Treasure-seeker" (an illustration for which graces the title page of the volume). The story begins as an old man recounts a story from his earlier life. He was a destitute child--a begger--to whom appears a spectre calling himself "the Treasure-Keeper of the Harz." This spirit requests the storyteller follow him to find a treasure. Tell me this passage doesn't sound like something right out of a game...
"Proceed...towards St. Andrew's mountain, and there enquire for the Black Kin's valley; or as it now called, the Morgenbrodsthal. When arrived at a brook, named the Duder, follow its track, against the current, until thou reachest a stone bridge, hard by a saw-mill. Pass not, however, over the bridge, but still continue to advance with the stream on the right hand, until thou seest before thee a steep rocky crag. A bow shot distance from this, though wilt perceive a hollow, resembling a grave prepared for a dead body. Do not fear, but clear it without apprehension, although thou wilt find it no very easy labour: thou wilt perceive that is has been filled up with earth intentionally. Having now discovered a stone wall on either side, proceed manfully in thy work, and thou wilt soon meet with a square flat stone, built into the wall, and about a yard in height and breadth. This being wrested out, thou wilt be at the entrance of the vault where the treasure is deposited."
The spectre's description goes on to pretty much flesh out the vault for the DM, noting where the wrong turn leads to vipers, and where not being prepared will extinguish lanterns, etc.
Not to mislead you, the entirety of the story is not like this. Nor are all of the stories. In fact, an 1823 review appearing in Blackwood’s Magazine described Popular tales and romances of the northern nations as "a disappointing publication more likely to do more harm than good to the cause of German literature, there being no discrimination in the choice of the pieces...and little sign of competence in the [anonymous] translations."1 But, from what I've read so far, there are some okay reads here, and the occasional kernel of an idea for game use. Note, however, some of these stories appear in better versions elsewhere (e.g., "The Treasure Seeker" appears in Andrew Lang's Crimson Fairy Book, and is a much better-written version, not to mention it's illustrated by OSR-public-domain-favorite Henry Justice Ford.)
1. De Quincey's Gothic Masquerade. By Patrick Bridgwater. Amsterdam and New York: Rodopi. 2005.