This is intended to be a lite guide that focuses on a number of important factors that tend to be misunderstood about this particular round type. A full guide regarding the "Devil" round type can be found here: https://tgstation13.org/wiki/Devil however I will attempt to talk about some more practical things.
What is the "Devil" round?
The Devil round is generally a more RP focused event than most and can be coupled with numerous other events such as Syndicate, Cultists, etc.. The basis of the round is that a player is chosen to be the "Devil" who is responsible for CONVINCING players into accepting contracts. If a crew member accepts a contract they will be given powers in return and the Devil's Soul counter will increase by 1. While it sounds like a win - win, the crew ultimately suffers for accepting contracts from the devil. As the devil gets more contracts signed it will gain more power and abilities to the point that, if they gain more than Nine Souls the Devil will ascend and be poised to take out the station entirely.
How do I know if there's a Devil on-board?
Aside from someone screaming about a giant "Thing" on the station; there are a number of ways to detect that there is indeed a devil on the station.
If you see a devil, it's generally a good sign.
If someone speaks of having a contract. If found, a contract from the devil will appear to be constantly burning but will be unable to be destroyed by normal means. All attempts to burn or douse the paper generally ends in failure.
Numerous crew members using powers such as: rampant hulkism, ability to make valuable things out of thin air, AI protecting random crew members and treating them specially, magic powers such as "lightening bolt" (which acts like a tazer), x-ray vision, presence of ghosts, and the dead returning back to life spontaneously (as a devil can offer a contract to a dead person as a means of reviving them at the cost of their soul.)
If the Librarian has a strange book called: Codex Gigas.
I know there's a devil on board, what can I do?!
The following steps should be taken in roughly this order:
Do not take a contract with the devil and encourage others not to do so. Greytide are generally the bread and butter clients of devils due to them being easy to convince.
If you sign a contract, tell IA, the magistrate, or the HOP. If you are the HOP, IA, or Magistrate it is advised that you remind them that the crew's soul was already signed over to NT. This can be used to buy back a soul from the devil, thus reducing the devil's soul counter by 1. All crew contracts can be found in the filing cabinet in IA.
The chaplain, librarian, security, and captain should be informed of the presence of a devil immediately. Give the librarian or chaplain one of the devil's contract as this will allow them to research the name.
If you are the librarian you are responsible for researching and informing the crew of the Devil's name, his weakness, and how to kill him. You are the Only person who has a 100% chance to get correct information from the Gigas as other roles are likely to get misinformation if they look it up.
Make sure to use the Devil"s TITLE and NAME or you will not get correct information. If they are Dark Lord Bob, you need to research "Dark Lord Bob" as it is written in the contract. Simply putting "Bob" will give you an entirely different rule set.
All information that the book gives you is vital to destroying the devil. If it tells you to wear white to "ward" off the devil, then wear white when hunting the devil. Do not skip steps.
Due to the nature of the Gigas, every time you read it you risk brain damage. IPCs appear to be immune to this effect.
After getting the information from the Librarian or other source via the Gigas, begin to prepare appropriately and follow All steps listed within the Gigas.
If you are Unable to kill off the Devil or Bind it per it's steps in the Gigas, repeated killing of the Devil will cause it to lose souls to the point that it will no longer be able to return to this realm.
The guide does a great job at explaining HOW to play as the Devil which I won't discuss here.