You possess a peculiar soul, rare and prized by the unearthly entities that like to toy with such goods. "Alpha soul", or "Puppetmaster's soul" are the most common nicknames used to define someone who has the ability to materialize the Einherjar, spirits that grow on items with long and/or particular histories.
However, calling and materializing such a soul is no easy task, and even an individual as peculiar as you does not have the power to give form to such beings. This is what the Ersatz are for, also called "Artificial spirits", they are condensed divine power that can host an Einherjar and create an appropriate form for it, which is then tied to the Alpha soul that summoned it.
... Or so it is said, but it's actually such a rare and undocumented occurrence that it's best left to bards' tales and folklore, for very few savants can actually claim to be experts on the subject and most of them are hardly human.