I should start by saying, Sertori are not scholarly wizards who painstakingly research their magic. Rather they are born with the natural ability to cast specific spells that are a part of their very soul. Each Sertori has a small fragment of the powers once possessed by the god Senga (see previous blog entry for details). As they grow in power, Sertori can become religious figureheads with followers and disciples.
In Sertorius magic is based around four emotions. The Emotions are the fundamental forces of creation, used by the gods when they shaped Gamandria: Agape (Love), Deimos (Fear), Penthos (Pain) and Misos (Rage). This is reflected by your Emotion Rank, which is 0-3 in each of the Emotions. Characters have four Ranks total, allowing them to take 1 rank in each or 3 Ranks in Misos and 1 Rank in Penthos. There are a number of possible combinations here. Your rank determines how powerful spells of that type are when you cast them, but it is a double edged sword because the higher your rank, the more risk you face when casting the spell at full potency.
Every Sertori begins the game with 4-6 spells (characters with one rank in each emotion gain 2 bonus spells giving them six). Sertori also have a Divinity Rating which increases as they gain followers, and this gives them more spells and access to miracles as they advance.
Each spell is keyed to a particular skill. So the spell Bridge uses your Trade (Architecture) skill for casting while the spell Splintering of Yaum uses Athletics. When casting a spell you can choose to cast it normally, which produces a nice effect but doesn't expose you to any risk, or you can cast it Cathartically. When you do the later, you unleash the full potential of the spell, handling the raw forces behind them and risking your body/mind.
When spells are cast Cathartically, you gain Grim points (an amount equal to your Emotion Rank on a Success, but equal to your Emotion Rank plus 2 on a failure--on a Total Success you take no Grim Points). If you take too many Grim Points then you start to undergo changes. This begins with minor mental and physical afflictions, then progresses. It culminates with the Sertori becoming a Grim: a twisted creature, a haunting force or something even more powerful.