How to Unlock Shrines
Before starting this sidequest, you must have at least one of Mreuva’s daughters alive and in your party to do this sidequest. If Cerya, Cistina, Ollivya or Sherri are all dead or otherwise didn’t join your party, you’re out of luck on this one. Don’t need all of them, but at least one.
Any time after Barnicia Castle in Ch4, check the Warren Report for the quest to save Lindl and do that. You’ll also need to check the talk topic “Mysterious Blast on Exeter” to unlock the path towards PotD. There’s a few story battles that go towards PotD but then diverge to Lhazan. Complete that sidequest, too.
From there, check the Warren Report again for the talk topic “The Ancient Temples”. Then, 6 forts will have battles (they don’t all turn red, though). These forts are Gecho, Lhazan, Qadriga, Boed, Geyld, Ndama. Once you’ve done the battles at all of those forts, go to Coritanae Keep and you’ll get a cutscene with Mreuva. Answer “I know I will” and the elemental shrines will unlock at those 6 forts.
Going to a fort will trigger a cutscene where one of Mreuva’s daughters will say a prayer to unlock the temple. Then you just proceed through it like a dungeon. The boss at the bottom will drop an elemental “apocrypha” spell, which is like a stronger but more expensive version of the tier2 overhead AoE elemental spells. They also drop a lot of crafting books. Once you’ve cleared all of the temples, the final temple boss will say a special message about acknowledging you as a “real oracle” or something and then drop oracle marks. These will let you turn the 4 sisters, Deneb and Iuria into shamans.
After you’ve completed this sidequests, the temples are all open for repeat dives. The boss of each temple’s loot table will have changed, and you can actually retreat from the battles now. This means you can farm tier2 apocrypha spells and in some cases rare drops like orichalcum.
Note: Gecho Fortress is south of Grimsby. Grimsby is unlocked by progressing through Lindl’s recruitment sidequest, which is triggered by reading the “Rhamsen Wreck Raised” Talk Topic in your Warren Report after the events at Barnicia Castle (and the subsequent cutscene at Phidoch).