Divine Classes are the strongest certifications in Fire Emblem: Fortune's Weave, but reaching them first requires five Keys of the Diadem hidden across Part 3's refreshed dungeons. The keys can be collected as soon as Part 3's Second Section begins, although their destination does not open until later.
Once the Temple of the Diadem becomes available, each key opens one sealed chamber containing the unique items needed for Divine Class exams. This guide explains exactly when the temple appears and lists every dungeon holding a key, allowing you to collect all five before the relevant story event and claim every Divine certification item immediately.
This guide contains spoilers for Part 3.
The Temple of the Diadem opens automatically at the beginning of Free Time immediately after the battle against Shenlo. This is part of Part 3: Salvation's Fourth Section, The Hopes of Monarchs and of People.
There is no separately named side quest to accept. The Temple simply becomes available on the World Map once the Fourth Section begins. However, you can start collecting its keys earlier: Part 3 dungeons receive new enemies and treasure chests at the beginning of the Second Section, and all five Keys of the Diadem are found inside these new fixed chests.
These keys each unlock a door in the Temple of the Diadem, with each door concealing one or two chests that allow you to access a single class:
Collecting an item does not immediately change a unit's class. It allows a compatible unit to take that Divine Class's certification exam, so you will still need to meet its weapon-proficiency requirements and achieve a 100-percent pass rate.
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