Image: UbisoftOfficial screenshot from Ubisoft.
Undying Tradition unlocks when you finish the Yaran Story Triada Blessings. It is worth doing for more than the trophy, since the quest also gives you the Triador Supremo and the Amigo Oluso.
Start Triada Blessings on Isla Santuario. Complete the Treasure Hunts for Ída's, Mimo Abosi's and Okú's relics in any order. Return all three idols to the Oluwa Cave to finish the story.
The relic hunts are spread across the map, so pick each one up when you are already working in that region. They lean on climbing and puzzles more than fights. Recruiting Oluso is a strong stealth upgrade, since she can hide bodies after a kill.