Elizabeth Boyer's 1980 The Sword and the Satchel, a Scandinavian-mythology fantasy, is a story of a fighter with a magic sword and a wizard with a bag of holding. It has a couple more D&D moves worth pillaging:
The frost giants... shouldered their cudgels and passed swiftly, uprooting a few trees for practice. Their glancing eyes filled the air with snow and their breath was like the coldest night in deepest winter.
This is a beautiful detail: just as snow at night is often only visible under streetlights, there are flurries of snow in a frost giant's cone of vision. It's also a useful signal for PCs who are trying to sneak by a frost giant and want to know if they're unseen. Except if it's actually snowing. Then they're screwed.
"A real lingorm!" Kilgore puffed. "Do they really get bigger with every bit of gold they have, or is that an old wives' tale?" "Old wives aren't so misinformed," the wizard retorted.
In D&D, there is a correlation between dragon size/HD and treasure size. It would be interesting if there were causation as well. It would make dragons' greed for treasure make more sense. It would also explain why Smaug freaked out when Bilbo stole that golden cup. That theft made Smaug diminish, just a little. (Maybe the missing cup caused the missing scale over Smaug's heart?)
If it seems too extreme for all dragons to get this lore, you could give it just to metallic dragons: a silver dragon's size is based on the amount of silver she has amassed, for instance. In this case, copper dragons would be totally safe. No one wants their hoard of a million copper pieces.
Three horsemen were riding up the side of the barrow toward a flickering blue light at the top. Kilgore strained his eye against the crack to see better, his heart suddenly thudding. From the ancient folklore of his people, he knew a blue light in a barrow mound meant there was treasure inside.
I don't know enough about Scandinavian folklore - is this a thing? Whether it is or not, it seems cool.
I wouldn't automatically put a visible blue light in every dungeon. I might turn this into a spell: someone using some sort of Detect Treasure spell might be able to see the blue glow that let them know this dungeon was worth exploring. It might even be a free-to-cast ritual known to all sorts of adventurous people.
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