Most players know what a demon is, more than likely arming themselves for the coming fiend adventure with an arsenal of stereotypical defenses. Certain players might even have read the source books, recalling stats and important resistances. 3rd Edition gave excellent monster descriptions, and 4th Edition supplied tables that helped inform player knowledge. 5th Edition has neither. It supplies you with heaps of lore to apply to your game but does not tell you want information is player ready.
One of your many jobs as DM is to control the knowledge that players have, possible through a number of avenues.
You let knowledge be readily available. Your world is full of taverns, guilds and libraries where the characters pick up information. Only your powerful custom monsters are unknown and undocumented. The rest can be easily studied. This is a very simple to control world with very little prep time because you read descriptions directly out of the Monster Manual.
You pull a Hollywood “Everything you know about Demons is wrong.” Your world is highly customized, and you've taken it upon yourself to create unique monsters or augment existing ones. The out of game knowledge players have is just folklore; your world works differently. Because there is intensive pre-campaign prep time, it lets you improv freely as you have no books to consult.
Everything is customized for your game. When you introduce new elements, it’s in a way that is controlled. To prevent confusion, you ask players to ignore out of game knowledge and to just go off of what they are given. This requires a heavy initial time investment with continued prep-work.
Imagine a campaign based around demons. In this world, the lesser are common pests responsible for outbreaks of sickness and trade route mauling. Perhaps the civilized people in such a land have dispensed leaflets or had a town crier give proclamations to advise the populous on home defense.
Demons come in many different types, spanning all 20 levels of 5th Edition Dungeons & Dragons. The entry at the start of the Demon section gives you an idea of the powers they possess but not how those manifest in the players’ eyes. Most entries even lack a physical description beyond the provided picture. The individual traits of each monster are lost on players, only to be found out by accident in combat.
For example, manes are the weakest demons, counting for 1/8th of a level. DM’s can read their lore and show the picture, but that doesn't tell the DM what information is ready to be shared with the players. In a world full of demons, what knowledge would the average person have? How do you transform stats and lore into something digestible?
While you know they are souls from the lower planes resistant to elements and immune to poison, the players don't. Give the players a description that highlights key features and makes players feel clever for translating. Key features, like dark vision and other stats, can be hidden in the guise of a town announcement.
Declaration 109: Mane Infestation
Manes have recently been sighted in this area.
Be advised there is now a curfew. No one is allowed after sunset.
They appear as small burned folk, often found in dark places of hot or cold. If you should sight one, report it immediately to the town guard.
These fiends rarely travel alone and have strength beyond their stunted bodies. They are dim-witted but can easily overwhelm in numbers.
We have reason to believe that they are immune to magics, though lemon juice is reported to scare them away.
There’s nothing wrong with a bit of misinformation thrown in. Manes are not immune to magic, though they do have an impressive number of resistances. No demon has resistance to acid and giving the lowliest one an aversion to lemon juice can help tip your players off and build a richer world. The lore section describes that manes die in a puff of foul gas and are reborn from it later. Let that be a surprise to ruin the first adventure’s success.
Quasists are another low level demon that can be treated in a similar way. You could inform players by having them discover a research journal.
Research on Quasit
One of the most common signs of a Quasit is repeat sightings of over-curious vermin. If you feel that a rat or crow has been watching or following you, a Quasit may very well be at work. In their natural form, they appear as 1-foot-high horned, frog-skinned bipeds with sharp talons and teeth.
Quasits are extremely poisonous creatures and direct contact should be avoided. Likewise, a case of multiple poisonings can mean a Quasit is at work. When attempting to capture suspected vermin, approach with caution.
Quasits are the lowest form of demon to enjoy corrupting souls. A Quasit sighting more than likely means there is an individual in league with the corruption. Quasits often make their lairs close to their summoner or host, spending their time living as vermin in closets, attics or inside walls.
Quasits can't resist a scandalous secret, particularly one that has the promise of power. The best way to rid yourself of a troublesome quasit is to make it believe a scandalous lie. It will then return to its home dimension to report to its master. If it does not reappear, we can assume it was punished for its foolish behavior.
Cowardice and deceit can be found in any mortal servant. I will never understand those who stoop so low as to employ a Quasit.
- Graham Fletwock
Using these methods, you can dispense lore to your players that keeps them engaged. These two detailed examples work best on low level monsters. With more difficult and rare enemies, limit knowledge further to keep the mystery and danger.