This much is real: In the late Renaissance and Early Modern era, Venetians mined in the German mountain ranges - primarily so that they could find rare minerals for their famous glass production. Since the details of these were considered state secrets, their mining operations were likewise fairly secretive.
However, Germans noticed these strangers nonetheless, and built a whole mythology around them. Folk tales and legends about magical "Venetians" abound in many of the German mountain ranges, and I will share the following example with you to show just how bonkers these could get:
Venetians on Blocksberg Mountain
Many years ago, three miners lived in Hahnenklee. From time to time, Venetians visited them who were searching for the treasures within the Blocksberg mountain, and usually they found enough that they went home contentedly. To keep up pretenses, they brought many medicines from their home which were effective against many illnesses. But the miners soon realized that they did not come for this kind of trade alone. Their main focus seemed to be directed at the Blocksberg, as each time when they arrived they first made inquiries whether any of their fellow countrymen had visited before them and visited the Blocksberg. If this happened to be the case, their mood always turned sour. But if none had been there before them, they seemed to be secretly pleased. Thus, the Blocksberg had to be important for them.
Once, they were visiting again, had enquired as before, and afterwards set forth towards the Blocksberg during a bright, moon-lit night. Then, one of the miners - who were friends with each other - followed them, and saw that they had dug the ground open at a remote spot at the Blocksberg. They then filled their bags with soil from this hole. The miner had seen enough, and memorized the location where this had occurred. He hurried back so that he would arrive home before the Venetians. The following morning, the latter departed, and the miners agreed to visit this location in the same evening. They wanted to investigate what could be gathered there, and already spoke of the riches that might be found. But the third miner declared that he didn’t want to accompany them, for whatever he was destined to receive would come into his household on its own.
The two other miners thus went there and searched at this spot, but didn’t find anything. They finally were about to give up and sourly gather their tools, when one of them took a strong swing at the ground and joyfully exclaimed: “There is something sticking in here!” Once more, they started to dig, and excavated a skeleton, but they were uncertain whether it belonged to a deer or a goat. If they were in a bad mood before, it worsened much more over this. But despite the anger, one of them still laughed at the other over this deception.
“Well,” said one of them, “our friend must also receive his portion. We should bring the skeleton into his house. He is down in the mine, his wife is in bed, the doors are open, and thus we will be able to put it into his chamber without being seen.”
Once night had fallen, they calmly brought the skeleton into the chamber of their comrade, and then went down into the mines. They still found their comrade during his labors, as he had made righteous efforts and his holes had been excavated deeply. When they arrived, he immediately asked:
“Well, are your barns full? I thought you would never have to put a hammer into your hands again!”
“Oh,” they replied, “cease your mockery! We would have fared better if we had gone down into the shaft.”
After that, they worked until twelve. Then they ended their shift, walked home together, and everyone went to their own abode. When the third one stepped into his chamber with a burning lamp, he was quite astonished. For everywhere in the room - on the window sills, on the chimney boards - there were splendid little figurines made out of gold and silver. There were stags, does, pigs, cows, calves, goats, birds, and so forth. He could not stop looking at them, weighing them in his hand, and marveling about the heft and beauty of these objects. After he had inspected everything, he went to bed and thought: “My wife shall wonder tomorrow morning about where these things came from as well.” But there was no trace of the skeleton.
The next morning, his wife woke up and stepped into the chamber. As she became aware of these riches of gold and silver, she immediately ran back, woke up her husband, and asked him: “Husband, where did you get all those beautiful things from?” But he replied: “The dear God has brought them into our house”, calmly turned on his other side, and continued to sleep. The woman locked and latched everything, and did her domestic chores. When it was nearly breakfast time, the man rose up and went outside in order to fetch washing water. At that moment, the two other comrades approached and merely wanted to receive their scolding for the prank. But instead of looking upset, their friend approached them in a friendly manner and spoke:
“Friends, it has occurred as I told you. God has brought me great fortune into my household. Come in, you shall have your part.”
Then he led them into the chamber. They became silent and rigid. Then he said: “You, friend, take this one, and you take that one. I shall take that one here, and then each has so much that he shall not have to hammer on the drill again.” Both of them thanked their comrade for this great gift, and finally asked: “What did you do with the skeleton?” But he did not react to this question, and only replied: “This doesn’t matter. Everyone should pack their riches together and carry them home.” These were heavy, so heavy that they were hardly able to move them. Later on, the three of them sold their gold animals in Goslar. The Duke of Braunschweig received a few as well, and the miners gained so much money from this that they became rich people and remained rich for the rest of their lives. From this time on, no one has ever seen a Venetian on the Blocksberg mountain again. The treasures within the Blocksberg are also sealed away until no four-legged animal has stepped on the Blocksberg for a hundred years. But it will be a long time until this has occurred.
Source: Wrubel, F. Sammlung bergmännischer Sagen. 1883, p. 91ff.
Playing in a D&D 5E game, running a Scion campaign heavily inspired by German folktales and folklore.