13 - Dreizehn (English: 13 - Thirteen) is a free German RPG. It plays in the modern world but adds occult elements. Players may take the roles of humans, watchers (humans that can spot magic), or magicians. Supplements also add half-angels and half-demons as character options.
Basic task resolution uses a d10 that is rolled against an attribute or skill. Conditional modifiers may be applied to change the target value.
Character creation is point-based. Points can be spent on the ten attributes, common skills, magical skills, and advantages. Disadvantages can be selected to gain character points.
The damage system differentiates between mental and physical damage. There are also hit locations. Each location has a damage threshold. When damage of a single hit exceeds this threshold, negative effects take place like unconsciousness, pain modifiers, rendering a location useless, etc.
A combat round is divided into five phases. The initiative rating (calculated from the attributes) defines in which of the phases a character may act. Every action has a certain duration measured in phases. The outcome of attacks is determined by the basic task resolution mechanic with the addition of combat modifiers. 13 has also a system of melee attacks and defenses, including martial arts techniques.
13 features a free-form magic system that presents some outlines to find out the difficulty of a magical effect. Casting such a spell causes mental damage. If it exceeds the mental hit points, it also causes physical damage, possibly killing the caster. The game presents various cults of magic with different advantages and disadvantages.
Characters also have an insanity rating that increases if they encounter disturbing sights or actions.
Character advancement is done with experience points that are awarded by the GM and may be spent to buy attribute or skill increases.