Foreshadowing is an extremely common element in the fantasy genre at large. At the same time, information can be extremely valuable to the players. Some creatures in particular are much harder to fight if the party is not prepared for them. From ancient prophecies to captured enemies, there are many different ways to give such knowledge to the players. I hope to present multiple examples of how this can be achieved as well as bring up some issues associated with them. I will do so by mentioning large concepts that are present any time that information is given to the player and how they are important. I'm just going to say right here that as always, finding the right balance for the group is important.
When players are given information can play a very important role. If information is provided too early, the players may forget that they were even told something important and if it is too close to the task that requires it, the players are more likely to connect the dots. Depending on the task being attacked, the situation the players are in and the actions of the players, any length of time could be valid. However, there are also times where it will be impossible to obtain certain information. If the players are actively looking for information about one kingdom attack another and there are currently no plans to do so, it stands to reason that there won't be any documents for such a thing in the general's quarters (still technically possible if there is a backup plan).
Can the information be trusted? Obviously, if every bit of information the Dungeon Master gives the players from a town drunk is incorrect, they won't trust a single thing the drunk says. At the same time, if every word one of the gods says is true, they will believe every bit of information that is told to them by that god. Most people in the world are probably less reliable than a god but know at least something that is true. If the players are being told a story by an NPC, it stands to reason that some of the details or elements might be wrong even if most of it is true. Finding a good balance for the reliability of information that players receive will then play a massive role in both the players' ability to tackle problems as well as building the world in general.
Who here has ever read a confusing prophecy? In such a case the meaning of the information is usually known just prior to when the characters need it or after they've already solved the problem. Seeing my players infer information based on context is a good thing. However, sometimes the information players receive will be far more obvious. It's generally hard to misinterpret a villager telling you, “Yeah, there's a cave over there. I pick mushrooms by it all the time.” At some point, being too far into one extreme can be extremely annoying. No one wants to get a cryptic prophecy when trying to ask a guard if there are any weapon shops in a city (excluding some joke games, of course). As always, finding the right balance for the group is the key.
How specific the information the players receive will also play a role in how they approach a problem and shape events. If the players are told that the magic thingy lies in this 50km square, their actions will be completely different than if I told them exactly the building, room and nook it was hidden in. This is a difficult section to fully explain since there are many different elements that make it up. If the party is looking for a particular person, we could know exactly where they are, exactly why they are there, why they are still there, where they were before, what their favourite weapons are and much more. Each one of those sections could be addressed differently (maybe we have no idea where they came from or what they are doing, but were able to track them down to a city via rumors).
- Prophecies (can be ancient prophecies about soon to occur events, ancient prophecies about events centuries away or fresh prophecies about either)
- Sounds in a dungeon (hearing the flapping of wings beyond a door gives the players certain information)
- Town gossip
- Recruited spy
- Captured enemy (the higher the rank, the juicier the information)
- Old book studying the same item you are now studying
- Corpse stuck in a trap giving you the hint that there is a trap
- Smell coming from an empty room
- Erie feeling (necrotic magic in particular may give sensations when close)
- Heat (especially when adjacent room is on fire)