Mythic items are one piece sets that you can earn from the new Antiquities System.
Mythic items were teased during the Greymoor Chapter Announcement stream. Mythic items are a new type of sets that are unique. They only have one bonus, but also only require you to wear one piece. In fact, you can only wear one Mythic item at a time. It looks like these new Mythic items will be very powerful for both PvP and PvE, close to the strength of a normal set five piece bonus or the unique bonus of a Monster Set.
Mythic items will obtainable through the Antiquities System. Antiquities can be found all over tamriel, base game, DLC and Chapter zones. Therefore, if you want to get all of the items you most likely need to have all Chapters and DLCs. That sounds like a lot, but usually if you buy the newest Chapter you get all the old ones for free and the DLCs can be obtained by simply having the ESO Plus sub.
The Mythic items so far might not sound that impressive, but we are unsure how high their stats will be at the end and lets not forget that they are one piece sets.
So far we only know about two existing Mythic items:
Mythic Item 1
1-Piece Bonus: 20% extra movement speed outside of combat, 10% extra movement speed during combat.
Mythic Item 2
1-Piece Bonus: While blocking, regenerate magicka.