For all of human history, magic has leaked into the world unpredictably. Often benign, a few unwittingly mastered techniques that harnessed magic to devastating effect--and they were the only ones capable of holding off magic's darker manifestations. Living entities caught in the area when the leaking turned into a sudden burst, impervious to all normal harm, they got known across cultures for their endless desire to consume, ultimately getting labelled as 'Voids' in scientific consensus.
The leftovers from their bodies always took the form of some innocuous material--a lump of pure metal, or a flawless piece of timber--and inevitably items were made consisting only of these remains. Idols, weapons, instruments... to most these were simply normal items but for a certain few these objects could be activated and draw upon magic to become armament and protection with abilities beyond even the greatest of martial arts techniques.
Defeating the Voids gradually fell to those who could activate these implements exclusively and the techniques faded from memory, much as the weapons themselves tended to; the focus of the bursts would move and via some intelligence the weapons would be 'lost', only to reappear each time. Some survived in legend, such as the Sword in the Stone or Excalibur, whilst others became nameless ornaments.
Whatever powered the bursts died down just as the scientific method was coming into practice, for centuries consigning the idea of magic to history--treating Newton's attempts at alchemy as a waste, rather than one of the last historical processes to create a relic. Yet a remote accident in the Arctic circle alerted the scientific community, then governments, to the truth. The bursts stayed rare whilst a scientific attempt at handling this was attempted... leading to the first new creation of relics in centuries, though no progress on working out how to force them to work with people (such as trained soldiers).
The bursts have moved to Tokyo, forcing the international organisation SAGA* to move with it. Their resources are limited--a mere trio of already-paired girls, and some relics that have found no matches, in a city with a high risk of casualties and the Voids are acting more intelligently than previously observed. At least in something of this size, the chance of relics already having found their user is high...
*Scientific Artefact Gathering Agency