I'm not sure if you answer this level questions @JeremyECrawford but our party now has possession of Tearulai from DotMM and would like to know the save DC of it's polymorph spell. Also, @DnDBeyond it doesn't seem to have a magic item entry.
— cmgriff (@cmgriff) February 10, 2020
If you cast a spell from a D&D magic item and its spell save DC isn't specified, the DC = 8 + your spellcasting ability modifier (which might be 0) + your proficiency bonus.
— Jeremy Crawford (@JeremyECrawford) February 10, 2020