Secrets and clues are short pieces of information the characters might discover during an adventure. Secrets and clues are initially written without regard to the method by which they might be discovered. Instead, we improvise their discovery as the characters engage with the adventure's locations and NPCs.
The following prompts don't create specific secrets and clues. Rather, the questions they ask are meant to inspire the creation of your own secrets for your campaign. Keep in mind that secrets are meant to serve you. Don't overthink them or worry about making them perfect. There's no wrong way to use secrets and clues as long as they help you run your game.
Use character secrets to tie the characters to the world around them. These sorts of secrets might be revealed by NPCs, old journals or letters, suddenly recalled memories, or prophetic dreams.
Use historical secrets to give the characters meaningful and useful information as they explore the setting of the campaign. Secrets of this kind should provide characters and players alike with bite-sized pieces of local or world history.
Historical secrets might be found as mosaics in ancient tombs, statues in old ruins, dusty tomes in ancient libraries, markings on strange weapons, or tales shared among elderly villagers.
Use NPC and villain secrets to reveal information about these NPCs to the characters, especially as a means of introducing villains before they face the characters.
Characters might learn NPC or villain secrets from a villain's herald or sidekick, rumors at a local pub, recovered journals, a minion's last words, captured letters, or town gossip.
Use plot and story secrets to teach characters about the larger events going on in the world, and to move the characters forward in the story of your campaign.
Characters might learn these secrets from quest-giving NPCs, notes found on defeated foes, dreams or portents from the gods, NPCs fleeing a disaster, arcane feedback from an object, or psychic projections.