Scath: Shadow of the Fomori uses a D20 based rule system, with many elements that will be familiar to a lot of experienced RPG players. There are six main "Attributes" (Brawn, Brawl, Speed, Mind, Charm and Mystic) governing the physical, mental, social and magical powers of a character. Derived from these attributes there are about two dozen "Skills", things like Sneaking, Crafting, Climbing, Riding, Diplomacy etc that are used to determine how well trained a character is in all these things. As a fourth element there are "Feats", allowing a character to break some of the general rules, do certain actions that would not be possible to him otherwise or giving him certain bonuses to skills or attributes in various circumstances. There are also some parts of the rules which are more unique. Every character is both fighter and mage at the same time, with eight different "domains" of magic you can chose from. Even without magic there is a wide variety of special abilities.

Focus Points allow characters to put all of their efforts into certain tasks, making them more likely to succeed.

Team Points are suited for combat as a group, allowing characters to help each other out even when it is not their turn or perform powerful combo – attacks by acting in unison. This is especially useful against bigger and stronger opponents.

Combat Styles are different fighting styles characters can specialize in, allowing them to perform advanced attacks and actions during a battle.

Destiny Points and Ascendence Points both symbolize a character's close connection to his Patron, a powerful spirit of nature, and allow them to perform miraculous feats, create magical artefacts or even come back from the dead!

The combat system is action based, fast and brutal, with weaker fighters sometimes dying after they took the first hit. With combat styles, basic actions, team actions and the "Grants", spell like abilities that every player character has, there is a lot you can do in combat, and a lot of interesting combinations to explore. The rulebook also contains rules for battles between hundreds or even thousands of fighters. These will be used later in a campaign, when the player characters are leading armies against the Fomori.