Read as a contract dispute, this is the clearest story the Norse ever told about themselves. A supplier makes an offer. The buyers alter the terms until they believe the job cannot be done, sign anyway, and then discover that it can. What they do next is not clever. It is fraud, followed by murder, and their own poetry says so.
An offer with an outrageous price
The gods were early in their settlement. Midgard was made, the hall of the slain was raised, and their own stronghold was still open ground.
A builder arrived and offered to wall it — a fortification good enough that mountain-giants and frost-giants could not get in even if they had already crossed into Midgard. He wanted three half-years for the work. His fee was Freyja, and the sun, and the moon.
The gods went away and talked. What they came back with was not a refusal but a renegotiation, and it is worth spelling out because the whole story turns on it. He could have the job. He would have one winter, not three seasons. He would work alone, with no man's help. And if any part of the wall was unfinished on the first day of summer, he forfeited the entire fee.
He accepted on one condition: the use of his horse. The gods hesitated, and Loki spoke up in favour of allowing it, and they allowed it.
The horse
Svaðilfari was the reason the whole calculation was wrong.
The builder laid stone by day. At night he drove the stallion out for material, and the loads it brought back astonished everyone who saw them: the horse was hauling more rock than the giant, and hauling it faster. The wall went up, high and thick, seamless, in the depth of winter.
Three days before the first day of summer, the only thing left unbuilt was the gateway.
The gods sat down and had the conversation they should have had in the autumn. Who advised this? Who argued for letting the horse work? It was not a difficult investigation. They found Loki and told him plainly that he would die unpleasantly unless he found a way for the builder to lose.
A mare comes out of the wood
That evening the builder went out to the quarry with Svaðilfari, and a mare ran out of the trees and whinnied at the stallion.
The stallion understood the situation immediately, broke his harness, and ran. The mare ran into the wood, the stallion after her, the builder after them both, shouting. They were at it all night. No stone came in. In the morning the work stood exactly where it had stood, and the gateway was not going to be finished by summer.
The builder, seeing what had happened to him, lost his temper — and when he lost it, his disguise went with it. Whatever the gods had told themselves they were dealing with, they now had a frost-giant standing inside their unfinished wall in a killing rage.
Oaths had been sworn. They called Thor anyway. He arrived, and the first payment the builder received was also the last: the hammer came down and drove the skull into the shoulders, and he went to the cold place under the world.
Some time later, Loki bore a foal. It was grey, and it had eight legs, and it became Sleipnir, the fastest horse in any world, which Odin rode for the rest of his existence.
The gods' own poets say they were in the wrong
The continuous story is Snorri Sturluson's, in Gylfaginning, written around 1220. But the incident is older than Snorri, and the older witness is the interesting one.
Völuspá — the great prophetic poem, and the single most important text in Norse mythology — gives it two dense stanzas in its account of the first troubles. The gods take counsel about who had poisoned the air and handed Óðr's woman to the giant kin. Thor alone acts, swollen with rage, because he seldom sits still when he hears that kind of thing. And then the poem's verdict: oaths were broken, sworn words, pledges, every solemn agreement that had passed between them.
That is not Snorri editorialising. The oldest source treats the building of the wall as an act of divine bad faith, files it alongside the other original sins of the world, and lets it stand as one of the reasons the whole structure eventually comes down at Ragnarök. A pantheon whose foundational construction project is completed by cheating and killing the contractor is telling you something deliberate about itself.
The folktale problem
There is a well-known Scandinavian legend type in which a church is built by a troll or a giant who demands the sun, the moon, or the builder's eyes, and who is cheated of the fee at the last moment — usually by someone guessing his name. Finn the giant at Lund cathedral is the famous example; versions attach to churches all over Norway, Sweden and Denmark.
The overlap with Snorri's story is obvious, and it produces a genuine and unresolved scholarly argument. One position holds that the myth is the older thing and the church legends are its Christianised descendants. Another holds that Snorri, writing two centuries into Christian Iceland, shaped an older mythic fragment with a folktale he already knew. Völuspá's stanzas are the strongest evidence for the first position — the oath-breaking and the giant's death are certainly pre-Snorri — but the wager, the deadline and the horse are not attested outside Gylfaginning, and there is no way to date them independently.
Why sun, moon and Freyja
The fee is not a random shopping list. It is precisely the set of things whose loss would end the world: the two lights, and the goddess of fertility and increase. What the builder asks for is not payment but extinction, dressed as a price.
It also puts Freyja in the same position she occupies in the theft of Thor's hammer — offered to a giant by gods who find it easier to trade her than to solve their own problem. She is not consulted here at all.
Wagner, and what he did with it
Richard Wagner took this story for the first evening of the Ring. In Das Rheingold, the giants Fasolt and Fafner build Valhalla for Wotan on a contract whose fee is the goddess Freia, keeper of the apples of youth, and the gods spend the opera looking for a way not to pay. Wotan's dilemma — that his authority rests on the runes cut into his own spear, so that breaking a contract destroys the basis of his power — is Wagner's invention, but it is a remarkably sharp reading of what Völuspá actually says about broken oaths.
Modern retellings that treat the episode as a caper, with Loki as the lovable fixer, lose the point entirely. The Norse version is not a heist. It is a record of liability.
Things worth clearing up
Was Loki forced into the shape of a mare? The texts do not say he was reluctant. He is threatened with death, he promises a solution, and the solution is the one he chooses. Norse tradition treats his shape-changing as routine and his sex-changing as a standing insult — Odin and he trade accusations about exactly this elsewhere — but nothing in the narrative suggests coercion once the plan is his.
Is Sleipnir's eight-leggedness explained by the story? Not by anything in it. The foal's shape is simply reported. Comparativists have linked eight-legged mounts to funeral biers carried by four bearers, and there is an eight-legged horse in Sri Lankan folk material that gets cited in this connection; none of it is demonstrable, and Snorri offers no explanation at all.
Did the wall get finished? Yes, but the sources are casual about it. The gateway is the unfinished part, and nothing says who eventually closed it. The stronghold stands complete in every later story.
Why does the builder's true nature only appear at the end? Because the gods' defence rests on it. Once he is revealed as a frost-giant, killing him becomes — in their own account — a security matter rather than a breach of contract. Völuspá declines to accept that defence, which is what makes the two sources worth reading against each other.
More on the fortress itself at Asgard, on the horse at Sleipnir, on the wider kindred at giants, and on how the whole system is arranged at the Norse tradition hub and the Norse family tree.