This analysis focuses on historical fact; for legends, please see here.
The metal composition of the blade is wholly inconsistent with the state of Celestuan metallurgy and thaumaturgy around the time of Sir Thomas Alexander of Estua or any period before. The sword is forged of an iron alloy not brought to the region until at least three centuries after Sir Thomas's death. More telling, however, is the use of the Nightstalker Runes which had not found use in any region until the discovery of the craft of runesmithing on the Iberian Peninsula.
Thus, at its earliest, the blade could not have been wielded by Sir Thomas because it could not have been created until hundreds of years after his death. The techniques binding magic to cold steel, difficult in the extreme, do point to Celestuan origin. Runecraft, however, has never been popular in that region even in its heyday elsewhere. This poses a puzzle it is difficult to unravel. Most likely, an early Iberian runepriest traveled across the plains and desert to the Arcades where he plied his skills despite the cultural taboo. Bring the knowledge of runesmithing into Celestua is easy; the alternative, a Celestuan weaponssmith bringing her craft to the peninsula or elsewhere, is difficult in the extreme. The abundance of metal-rich ore in the greater Celestuan area made smithing there substantially easier than it is anywhere else.
The first authenticated account we have of the Great Sword is not until centuries after its supposed gifting to Sir Thomas. The sword was included in the list of the Princess Orithia's dowery for her ill-fated first marriage to Phillip, the King of Ulam. Upon his death, the Great Sword was taken into the royal coffers and thence to his firstborn son.
Shortly after Godking Mandri's accession, the Sword – like all legendary weapons – became official property of the throne under the Legendary Weapon Edict though it is unlikely he ever wielded it himself or gave the right of its use to any of his lieutenants. In fact, it is unknown if this ownership was anything other than a technicality as the edict was only ever enforced sporadically.
At present, the Great Sword is on indefinite loan to the Condinnori Museum from Celestua. Some from the Order of South Star of Estua have sought return of the sword, citing the legend of Sir Thomas Alexander of Estua and its later theft despite the historical inaccuracy of these tales. The museum has deferred to the Celestuan Ministry of the Interior who has called the Great Sword "an important historical treasure that we hold in trust for the Celestauan people." The Ministry has made no official comment on the Order's repeated requests.
In addition to being a fine weapon in its own right, and crafted of Celestuan steel which will never dull or rust, the Great Sword has two remarkable properties. It will never harm the innocent, even by mistake. More than once, this has been used to test whether someone is truly innocent though since nearly every adult has strayed and the Sword is ambivalent of whether someone is guilty of the accused transgression or guilty in a broader cosmic sense, this use rapidly fell out of favor. The last major use of trial by the Great Sword was the Trial of Mozhgan.
Second, upon utterance of a command word engraved on its hilt, the blade becomes engulfed in silvery spirit fire. This fire has been called "bane of demons" in some early descriptions of the blade due to the effect it has on those effected with possession or of demonic parentage. Even presence in the same room with the spirit fire is said to cause a demon extreme distress if not actual harm.