This is unlocked by doing the following:
-Complete the Summerset main quest for The Good of the Many
-Collect all of the Psijic relics and return them to the College of Psijics Ruins for Relics of Summerset
-Complete 33 quests for Summerset Grand Adventurer
-Defeat all 6 World Bosses for Defender of Summerset
-Clear all 6 Delves and discover all Striking Locales for Summerset Master Explorer
-Complete the Karnwasten Group Event
-Complete the Sunhold Group Event
Use this map to see where all relics, group events/world bosses, delves and quests: https://esomap.uesp.net/?centeron=Heart+of+the+Indrik&wo...
Everything is very easy to solo besides the world bosses (even the world bosses can be done solo with a top tier build/set), Summerset is usually busy enough to find people to do them by posting in zone chat, especially during the Summerset Celebration event.
If there is any questions let me know, or if you need help message me, I can solo the anything for you, including the world bosses.