The lore you need to know before Tears of the Kingdom.
http://www.nintendoworldreport.com/video/63558/legends-of-hyrule-part-2-the-sheikah-and-the-yiga
What caused the most loyal protectors of the royal family to turn against them? In this episode we explore the history of the Sheikah and the formation of their extremist sect, the Yiga Clan.
