Then spoke Gangleri: “Whose is the horse Sleipnir? And what is there to tell about it?”
High said: “You do not know details of Sleipnir and are not acquainted with the circumstances of its origin!—but you will find this worth listening to. It was right at the beginning of the gods’ settlement, when the gods had established Midgard and built Val-hall, there came there a certain builder and offered to build them a fortification in three seasons so good that it would be reliable and secure against mountain-giants and frost-giants even though they should come in over Midgard. And he stipulated as his payment that he should get Freyia as his wife, and he wished to have the sun and moon.
“Then the Æsir went into discussion and held a conference, and this bargain was made with the builder that he should get what he demanded if he managed to build the fortification in one winter, but on the first day of summer if there was anything unfinished in the fortification then he should forfeit his payment. He was to receive from no man help with the work. And when they told him these terms, then he asked that they should permit him to have the help of his stallion called Svadilfæri. And it was Loki that was responsible for this being granted him. He set to work the first day of winter to build the fortification, and at night he hauled up stone with the stallion. And the Æsir thought it a great marvel what enormous rocks this stallion hauled, and the stallion performed twice the deed of strength that the builder did.
“But at their agreement there had been mighty witnesses invoked and many oaths, for the giants did not think it safe to be among the Æsir without a guarantee of safety if Thor were to return home, but at the time he was gone away into eastern parts to thrash trolls. And as winter passed the building of the fortification advanced rapidly and it was so high and strong that it could not be stormed. And when summer was three days away then he had almost got round to the entrance of the fortification. Then the gods took their places on their judgment seats and tried to think of what to do and asked each other who had been responsible for the decision to marry Freyia into Giantland and to spoil the sky and heaven by taking away sun and moon and giving them to giants.
“And there was agreement among them all that he must have been responsible for this decision who is responsible for most evil, Loki Laufeyiarson, and declared he would deserve an evil death if he did not find a scheme whereby the builder would forfeit his payment, and they offered to attack Loki. And he, being afraid, swore oaths that he would manage things so the builder would forfeit his payment, whatever it cost him to do it. And the same evening, when the builder drove out for stone with his stallion Svadilfæri, there ran out of a certain wood a mare up to the stallion and neighed at it. And when the stallion realized what kind of horse it was, it went frantic and tore apart the tackle and ran towards the mare, and she away to the wood and the builder after them, trying to catch the stallion, and these horses ran around all night and the building work was held up for that night. The next day not as much building was done as had been the case previously. And when the builder realized that the work was not going to be completed, then the builder got into a giant rage.
“But when the Æsir saw for certain that it was a mountain-giant that they had there, then the oaths were disregarded and they called upon Thor and he came in a trice and the next thing was that Miollnir was raised aloft. Then he paid the builder’s wages and it wasn’t the sun and moon, instead he stopped him from living in Giantland and struck the first blow so that his skull was shattered into fragments and sent him down beneath Niflhel.
“But Loki had had such dealings with Svadilfæri that somewhat later he gave birth to a foal. It was grey and had eight legs, and this is the best horse among gods and men. Thus it says in Voluspa:
Then went all the powers to their judgment seats, most holy. gods, and deliberated upon this, who had tainted all the sky with darkness and to die family of giants given Od’s beloved.
Oaths were gone back on, pledged words and promises, all the solemn vows that passed between them. Thor achieved this alone, bursting with wrath. He seldom sits idle when he learns of such things.”
– from the “Gylfaginning” in the Prose Edda,
translated by Anthony Faulkes, Edda, 35-36