Ever wanted to add a legendary artifact to your campaign, but you were worried about PC’s getting their hands on it?
Or you put the Legendary item in your game and the PC’s already managed to get it?
What to do, and how to fix it.
A casual remark by one of the party about something met on a previous adventure leads to a summons to and grilling by the Mages' Guild (or equivalent), probably resulting in an expedition back to that area to recover the things for 'study' by the Guild. The party's presence will be 'requested' for this expedition.