Fire Emblem: The Age of Legend
Mystery is a sword imbued with Dark Magic theorized to exist by many historians and religious scholars but unknown to most others. Its origins are uncertain, as is most of its history and indeed its true name. It is commonly attributed to some worker of Profane Magic, probably a believer of Azrah, and was probably made sometime in the Time of Sorrows according to this hypothesis by a fanatic fighting for his insane beliefs against the growing tide of Fundin. Grimholt was no doubt invoked in creating it, a very poor choice.
While said to be a powerful weapon indeed, there is a terrible curse (or perhaps a blessing twisted by the wiles of the inscrutable god) upon it. The sword draws upon the Aegir of the wielder for its power, moreover it is erased from history and perhaps from memory as well. Thus the sword is only theorized to exist, there are a sizable number of odd gaps in the historical record in which a notable figure seems to be wielding an artifact of great power, yet the weapon in question is unknown. The existence of Mystery and it having this strange property would conveniently answer many of those questions, on the other hand other explanations are still quite possible.
Its current whereabouts (if it exists) are unknown, however a few notable scholars and theologians from what are now the Koran Free Cities suddenly wrote much of what is known or guessed at regarding Mystery in the 2A 1600s and since then there have been few supposed uses of it. This leads some to suspect that this dangerous and unholy weapon is being kept under lock and key in secret in the Koran Free Cities somewhere, perhaps in a church reliquary. Assuming that it is indeed progressively erased from history, any record of it being hidden is of course non-existent.
Some speculate that the only reason any of the above information can be properly written down about such a sword is that it is all or nearly all incorrect and therefore is not erased by the weapon’s side-effects.