Spell Mastery is an attribute and gameplay mechanic introduced in Total War: Warhammer III. As an attribute it indicates the spell intensity of a spellcaster unit and as a gameplay mechanic it increases the potency of spells.
Spell Mastery can increase the intensity of spells cast with Winds of Magic. Spells normally have an intensity of 100%, but spell mastery can increase this up to a maximum of 200%. At 200% spell intensity, the effects of most damaging spells and spells that buff or debuff a target will be doubled. Also while Spell Mastery can make many spells extremely powerful, not all spells will benefit from high intensity. Spell intensity does not affect the duration, range, cost or cooldown of any spells.
Previously, access to spell mastery was fairly limited only to spellcasters with Greater Arcane Conduit or with the Mastery of Elemental Winds attribute (each additional spellcaster with this attribute in an army increases spell mastery by 15%; an army with eight of these casters will achieve the maximum 200% spell mastery) which is available to Grand Cathay, the Argwylon Wood Elf faction, and Kairos Fateweaver's personal army.
Following Update 5.0, Spell Mastery is now present in all spellcasters as an attribute. Units that only have Bound Spells do not get Spell Mastery.
A current bug with the UI does not show the effects of Spell Mastery on healing, most direct damage, and Winds of Magic recharge/reserve spell effects, but in-game testing[1] has shown that these spells are still benefitting from increased spell intensity.
Spell Mastery is an attribute and gameplay mechanic introduced in Total War: Warhammer III. As an attribute it indicates the spell intensity of a spellcaster unit and as a gameplay mechanic it increases the potency of spells.
Winds of Magic. Spells normally have an intensity of 100%, but spell mastery can increase this up to a maximum of 200%. At 200% spell intensity, the effects of most damaging spells and spells that buff or debuff a target will be doubled. Also while Spell Mastery can make many spells extremely powerful, not all spells will benefit from high intensity. Spell intensity does not affect the duration, range, cost or cooldown of any spells.
Grand Cathay, the 
Argwylon Wood Elf faction, and Kairos Fateweaver's personal army.
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