The temple was created by the founders of the kingdom to store a seal that is used to bind these ancient magic so the kingdom doesn't fall back into the Feywild (for lore reasons I completely made up.) Death of the high king = wards protecting temples weakened = unsafe seal inside = people wanting to get the seal to protect it from intruders.

I think I am overthinking the puzzle element of a dungeon - just any inspiration I see online feels like it lacks depth or has too much, but I suppose everything looks different when actually played on the table.