says Geoffrey of Monmouth, was erected by Merlin (the magician) to perpetuate the treachery of Hengist, who desired a friendly meeting with Vortigern, but fell upon him and his 400 attendants, putting them all to the sword. Aurelius Ambrosius asked Merlin to recommend a sensible memento of this event, and Merlin told the king to transplant the “Giants' Dance” from the mountain of Killaraus, in Ireland. These stones had been brought by the giants from Africa as baths, and all possessed medicinal qualities. Merlin transplanted them by magic. This tale owes its birth to the word “stan-hengist,” which means uplifted stones, but “hengist” suggested the name of the traditional hero.
Stonehenge, once thought a temple, you have found A throne where kings, our earthly gods, were crowned, When by their wondering subjects they were seen.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Stonehenge from Infoplease: