As are wont to be, there was a once a magic, and as magics are wont to be, it was sealed in a brand.
Some number of years ago, and the number is irrelevant here, there was some or other person of a presumably high stature and of great power who, upon seeing the potential dangers of a world with them absent, trapped a small portion of themselves in a simple brand which, through a simple ceremony of vows, could be transferred between the many humans of the world.
And with it came access to the power sealed within. It was through the potential held inside that its hosts were able to meld the flesh of their bones for the purpose of good, and sometimes of selfishness. As is also wont, it was often abused in its workings. Often, torture was the only mechanism to remove it and reassign the mantle to a more suitable guardian.
Naturally, it has come to today. A host of many years has worn themselves down on it, and are now seeking to engage in a transfer. And how might that go?