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Fortify Illusion Potion descriptions will read, "Illusion spells are <mag>% stronger for <dur> seconds." This allows targeted Illusion spells to affect higher-level creatures, and is applied before perks. For example, a Fury spell cast with a Fortify Illusion +50% potion and the Rage perk will affect creatures up to level (6 * 1.5) + 12 = 21. Without the potion it would be 18, an increase of 3.
On the other hand, a Frenzy spell cast with the same potion but no perks will affect creatures up to level (14 * 1.5) = 21. Without the potion it would be 14, an increase of 7. Thus this bonus is best used on high-level spells and can be effective even if the user does not have Illusion perks.
The following alchemy ingredients can be used to create a potion of Fortify Illusion:
Fortify Illusion Enchanting descriptions will read, "Illusion spells cost <mag>% less to cast." This reduction is stacked additively with other Fortify Illusion Enchantments and multiplicatively with other cost reductions. For example, an Illusion spell cast while wearing 2 pieces of -25% enchanted gear and the corresponding -50% perk will cost 1 - (0.25 + 0.25) * 0.5 = 25% of its full cost, times the skill multiplier.
On the other hand, an Illusion spell cast while wearing all 4 pieces of -25% enchanted gear will be cast for free, no matter what the other factors are because 1 - (0.25 + 0.25 + 0.25 + 0.25) = 0. This holds true for Fortify <Magic School> Enchanted gear of every kind.
The following items use this effect. You can learn how to enchant custom items with Fortify Illusion if you find one of the following items and disenchant it:
- All varieties of Armor, Helmets, and Robes of Illusion
- Includes the enchantments of Minor Illusion, of Major Illusion, of Extreme Illusion, of Eminent Illusion, and of Peerless Illusion.
Artifacts and unique items that use the effect but cannot be disenchanted include:
Vampirism grants the Champion of the Night effect which reads, "Illusion spells are 25% more powerful." This allows targeted Illusion spells to affect higher-level creatures but is applied after perks, making it very powerful for high-level Illusionists. For example, a Fury spell cast with Champion of the Night and the Rage perk will affect creatures up to level (6 + 12) * 1.25 = 22.5, rounded down to 22. Without Vampirism it would be 18, an increase of 5.
On the other hand, a Frenzy spell cast with Champion of the Night but no perks will affect creatures up to level 14 * 1.25 = 18. Without Vampirism it would be 14, an increase of 4. Thus, this bonus works best for characters who have invested in Illusion perks.
Keep in mind that dual casting an Illusion spell will double its strength—including any bonuses from Alchemy and/or Vampirism—in exchange for almost tripling the cost.