The Relic Crown is unobtainable in USUM source:
Bulbapedia
They probably could've obtained it by giving a Pokemon the Relic Crown then poke banked it over to USUM, but, unfortunately, the information I could find says that that's not possible as Pokemon lose thier item when put in poke bank. As poke bank is the only way to trade between USUM/SM and previous games, and the Relic Crown is a BW2 and BW exclusive item, I think it is hacked, as there is no other way of obtaining it that I could find.
