"Ladies and Gentleman, Children of all ages! Welcome to my Carnival! You will see sights to behold, and things beyond belief! Some may frighten you, and some may shock you, but its all in good fun! Please, stay a while…and watch."
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.