Demons
Characteristics
Non-corporeal entities that have escaped from Hell
Possess humans
Any injuries inflicted upon the possessed human, even if it doesn't injure the demon, will affect the human once the demon has left.
Possessed humans may be able to recall everything the demon did while they were possessed
Can be identified by black, white, yellow or red eyes, although eyes can appear normal if they choose
Cannot be hurt by regular means - they are immune to guns and knives etc
Presence often leaves a trace of sulphur
Some Demons were once humans who offered/sold/lost their souls to another demon
Others are Angels who have fallen. Not all fallen Angels become demons though.
Vulnerabilities
There are several methods that can be used to trap or slow down a demon:
Salt - They cannot cross a line of salt
Iron - They cannot cross an iron line, and can be hurt with an iron rod
Exorcism - Removes them from the host human and returns them to Hell
Holy Water - Burns them like acid
Palo Santo - Burns in a similar way to Holy Water
Devil's Trap - A symbol that is marked on the floor or ceiling that can trap a demon within it
Hallowed Ground - A low-level demon cannot enter a church or other sanctified area
But only two can permanently destroy them:
The Colt
Ruby's Knife