A certain farmer once lived at Reynir, near the coast of south central Iceland. He was ordered by the local bishop to build a good church near his farmhouse, but had much trouble securing the necessary supplies. Since he was also unable to find proper builders to help him, he feared that he would not be able to complete the Church before winter.

One day as he was walking in his field, thinking sadly over the matter and vainly attempting to find a way to excuse himself from the obligations of the Bishop, when he happened upon a very strange man. He had never seen this man before, and meeting him on that fateful day, they struck up a conversation. He told the strange man of his troubles, and the man promptly offered his services in building the Church, declaring that he should require the services of no other workman.

The farmer was skeptical and asked what the strange man wanted for payment. The man made the following condition: "that the farmer should either find out his name before he had finished the church, or else give him his son, who was then a little boy six years old". The farmer thought this to be a simple matter, and, holding back laughter, agreed to the strange man's terms.

So the strange mason set to work. He laboured intensely, day and night, rarely speaking to anyone until the Church began to rise beneath him as if by magic. The farmer was shocked at the amazing rate at which the mysterious mason worked, and began to worry, as what he had thought to be a simple matter was proving to be much more difficult. He asked everyone one he could and searched the contryside all around, but he could not, for the life of him (or, rather, his son), find out the name of the speedy mason.

On the day in which the Church was to be finished, the farmer was once again wandering his fields, brooding about the price it looked like he would be paying. He threw himself onto a grass mound in grief and eventually fell asleep. It was then that he thought he heard a voice, as of someone singing:
Soon will thy father Finnur come from Reynir,
Bringing a little playmate for thee, here.

It sounded like a mother lulling her child to sleep, and the words repeated a number of times. The farmer quickly guessed the meaning of the mysterious verse and then, his heart filled with joy, the farmer ran as fast as he could to the church, where he found the builder nailing the last plank over the altar.

"Well done, friend Finnur!" he said, "How soon you have finished your work!"

No sooner had these words passed his lips than Finnur, letting the plank fall from his hand, vanished, and was never seen again. The farmer, his wife and son lived happily ever after.
This is but one tale of a mysterious master builder. Oddly enough, unlike many folktales, the tales of the master builders are very specific, often specifying exact locations and specific churches. In some stories the Devil is the mysterious builder; others tell the story of why a certain church remains unfinished. Often, in stories where the builder enlists the help of the Devil, then tricks the Devil out of his expected payment (usually a soul), the completed Church is considered cursed and usually meets with disaster as a result. The church at Reynir did not meet with a sinister fate, however, and can still be seen here: http://www.eyeoniceland.com/photos/pom/reynir.html

Source: Jón Arnason, Icelandic Legends, translated by George E. J. Powell and Eiríkur Magnússon. Found in Folklore and Mythology Electronic Texts, edited or translated by D. L. Ashliman. (http://www.pitt.edu/~dash/mbuilder.html).