All spells in Trollslayers are given a Rank, which determines the idea skill (i.e. character level) of the spellcaster required to cast the spell, and an Essence Cost (EC), which shows the base cost in Essence the spellcaster needs to expend. For example, a spell of the third Rank might have an EC of 8.
For every level the spellcaster is above the Rank of the spell, the EC is reduced by one, though it can be reduced no lower than half the base EC for the spell (round down), and the minimum EC is always one. Example: a 4th level Magician casting the above mentioned Rank three spell would only have to expend 7 Essence points to do so, while any spellcaster of 7th level or higher would only have to spend 4 Essence points to cast the spell.
If a spellcaster is trying to cast a spell of a Rank higher than their character level, the EC is increased by the Rank number of each increment of Rank beyond the caster's character level. Example: a second level Magician trying to cast the above 4th Rank spell would have to expend a total of 15 Essence points to do so (base EC of 8, +4 because a 4th Rank spell is above the caster's character level (2nd level), and also +3 because there is the 3rd Rank of spells between the caster's character level and the Spell's actual Rank.
Spellcasting is divided into two forms, Sorcery and Ritual Magic. Some spells can only be cast by either Sorcery or Ritual Magic, while other spells can be cast in either form. Spellcasters can buy either, or both, Sorcery and Ritual Magic as talents to aid in their spellcasting ability. Both are PRE-based talents.
The effects of a given spells is defined by one or more of eight Aspects: Energy, Gaseous, Liquid, Matter, Mind, Spirit, Distance and Time. Spellcasters can take any Aspect as an KNO-based talent to aid in the use of spells defined by said Aspects.
The base difficulty for spellcasting is 8+ for spells cast via Sorcery, and 6+ via Ritual Magic. Magicians and Paragons add a +1 to all spellcasting. Any talents in either Sorcery or Ritual Magic are added as appropriate. Talents corresponding to Aspects that define a spell being cast are also added (if a spell is defined by more than one Aspect, the bonus is equal to the lower number of the relevant talents, or the highest elevant talent divided by two, round down). If the Magician or Paragon has a specialization that covers the spell in question (i.e. Divination, Weather Wizard, Necromancy), or an Expert's profession is appropriate to the spell (i.e. a woodsman casting nature magic, or a thief casting stealth-based magic), another +1 is added to the roll. Finally, the attribute bonus for either KNO or PRE, whichever is lower, is also added to the roll.
Magicians start off with KNO/3 spells of the first Rank, while Paragons start off with KNO/4 such spells. The maximum number of spells that any Magician can know at a given time is equal to the KNO attribute + PRE modifier + character Level. For Paragons, the number is half compared to a Magician, and Experts can only learn 1/3 the number of spells that a Magician could.