Illusion is a school of magic which affects the mind of either the target or the caster. It is the only school of magic with silent casting abilities for its spells.

Whenever an offensive illusion spell is cast, the game calculates a ”break-free chance” for the victim. There is more discussed on the Math behind this calculation within Magic Mechanics but in summary - Illusion spells can fail.

Several Illusion spells feature the upkeep mechanic. This means that you can keep a spell active for a small magic drain after casting. The proceed to cast further spells. Casting the spell again will cancel the spell and stop the magic drain.

Any illusion spell that contains either "muffle" and/or "invisibility" components is considered a hiding spell, which are all mutually exclusive. So yes, having Veil of Silence active and casting an invisibility spell will cancel the Veil of Silence.

Veil of Silence, Arcane Resonance and the Rune of Dampening are always cast without any noises NPCs could notice. So if necessary (as in "Intended by your playstyle") you can cast Shadow Stride or simple Invisibility to move into position unseen near an NPC, hide behind a corner with no line of sight to the target, cast a silencing spell and then cast whatever harmful spell you intended to invoke on the unaware target.

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Novice Illusion

You've acquired two Illusion spells from the Novice tier. Novice spells cost 55% less magicka, and improve 1% per skill level.

Level Required : 0

Perk Required : None

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Apprentice Illusion

You've acquired two Illusion spells from the Apprentice tier. Apprentice spells cost 55% less magicka, and improve 2% per skill level.

Level Required : 20

Perk Required : Novice Illusion

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Adept Illusion

You've acquired two Illusion spells from the Adept tier. Adept spells cost 55% less magicka, and improve 3% per skill level.

Level Required : 40

Perk Required : Apprentice Illusion

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Expert Illusion

You've acquired two Illusion spells from the Expert tier. Expert spells cost 55% less magicka, and improve 4% per skill level.

Level Required : 60

Perk Required : Apprentice Illusion

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Master Illusion

You've acquired one Illusion spell from the Master tier. Master spells cost 55% less magicka, and improve 5% per skill level.

Level Required : 80

Perk Required : Expert Illusion

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Acoustic Manipulation

You are now proficient at manipulating sound. Muffle, Arcane Resonance, and Rune of Dampening are 30% more effective, last longer, and can be dual-cast.

Level Required : 25

Perk Required : Novice Illusion

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Visual Manipulation

You've experimented with spells like Darkvision, Invisibility, and Blur to manipulate your surroundings. These spells are 30% more effective, last longer, and can be dual-cast.

Level Required : 35

Perk Required : Novice Illusion

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Environmental Manipulation

Environmental spells like Shadow Stride, Veil of Silence, Fade Other, Invisibility Sphere, and Muffling Sphere are 30% more effective, last longer, and can be dual-cast.

Level Required : 65

Perks Required : Acoustic Manipulation and Visual Manipulation

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Shadow Shaping

You know how to play in the dark. Shadow Shield & Shadow Simulacrum are longer lasting, more effective, and dual-castable.

Level Required : 70

Perk Required : Expert Illusion

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Phantasmagoria

You've mastered the most powerful forms of Shadow Magic. Therefore, spells like Veil of Shadows, Shadow Sanctuary and Mistress of the Dark can now be empowered and cast with highly decreased effort.

Level Required : 90

Perks Required : Master Illusion, Shadow Shaping, Environmental Manipulation

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Delusive Phantasms

You have a talent for magical manipulation. Fear, Calm, and Courage spells are more effective and can be dual-cast.

Level Required : 15

Perk Required : Novice Illusion

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Otherwordly Phantasms

Your illusions penetrate deeply into the minds of your foes. Hibernation and Frenzy spells are 30% more potent and can be dual-cast.

Level Required : 45

Perk Required : Delusive Phantasms

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Pain and Agony

Most in Tamriel have known pain, and you know how to press their scars. Phantasmal Pain and Waking Nightmare are 30% more powerful and can be dual-cast.

Level Required : 50

Perk Required : Adept Illusion

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Obliterate the Mind

Your phantoms dominate the weak. Mind manipulation spells like Fog of Shadows, Blackout, and Phantasmal Killer are 30% more effective and can be dual-cast.

Level Required : 70

Perks Required : Otherworldly Phantasms and Pain and Agony

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Domination

No thinking being can escape your influence. Phantasmal Miasma and Mind Maelstrom are more powerful and can be dual-cast.

Level Required : 90

Perks Required : Master Illusion and Obliterate the Mind

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Spells	Base Cost	Magnitude	Duration	Effect	Notes
Charming Touch	150	2	40s	The touched target considers the caster a good friend and stops all hostilities for up to <dur> seconds. If empowered, the strain drains <mag> magicka and stamina per second.
Clairvoyance	25/s	-	Concentration	The caster thinks of something that is to be found, and the spell will reveal the way.
Darkvision	50	-	86400s	With a whispered word and a magical gestures, the caster's eyes become able to reveal what lies hidden in the dark. Empowering the spell grants nightvision.	Upkeep: 2
Lionheart	150	25 (50)	40s	The caster's touch inspires valor. The target will not flee and gains <mag> extra stamina and magicka for up to <dur> seconds. If empowered, the target will also regenerate these attributes (mag)% faster.
Night Glimpse	5/s	-	Concentration	Dark Vision
Soothe (Rank I)	45	10	30s	Exposure is reduced by <mag> for seconds. When the effect wears off, the exposure is gained again.

Spells	Base Cost	Magnitude	Duration	Effect	Notes
Arcane Resonance	150	20	30s	The caster creates a loud noise at the target location, distracting nearby enemies. If empowered, the noise binds the attention of nearby targets to its origin, decreasing their ability to spot distant targets by <mag>% for <dur> seconds.
Blur	75	60	86400s	The caster becomes blurry, takes randomly decreased damage from ranged attacks and gains <mag> armor rating. Empowering the spell significantly improves the ranged damage reduction.	Upkeep: 5
Enraging Orb	250	25	30s	The target hit by this orb will attack anything nearby for up to <dur> seconds. Empowered: causes a tireless rage, increasing damage and stamina regeneration by <mag>%.
Hibernation Touch	250	30	40s	The touched target will fall into a deep slumber for up to <dur> seconds. Empowering causes an uneasy sleep, reducing magicka and stamina regeneration by <mag>%.
Invisibility	100	48	86400s	The caster becomes invisible until he interacts with the environment. The caster also gains <mag> armor rating and he cannot be aimed at precisely. If not empowered, the spell breaks if the caster runs.	Upkeep: 5
Muffle	80	30	86400s	The caster dampens all sounds around him or her, making movement practically inaudible for <dur> seconds and prevents the detection of the caster by detection spells. If not empowered, the caster can only move at walking speed.	Upkeep: 3
Rune of Dampening	275	-	600s	Places an arcane rune on the ground which absorbs all sound generated by nearby cast illusion spells. If not empowered, the rune also drains the spell's essence, increasing casting costs by 20%
Soothe (Rank II)	148	20	30s	Exposure is reduced by <mag> for seconds. When the effect wears off, the exposure is gained again.

Spells	Base Cost	Magnitude	Duration	Effect	Notes
Enraging Rune	400	25	30s	The targets crossing this rune will attack anything nearby for up to seconds. Empowering the spell causes a tireless rage, increasing damage and stamina regeneration by %.
Frenzy Sphere	400	150	30s	The caster unleashes an orb which will explode upon impact. Affected targets will flee in despair for up to <dur> seconds. If empowered, the targets are so terrified that they totally neglect their defenses and loose <mag> armor rating.
Frightening Sphere	400	150	30s	The caster unleashes an orb which will explode upon impact. Affected targets will flee in despair for up to <dur> seconds. If empowered, the targets are so terrified that they totally neglect their defenses and loose <mag> armor rating.
Hibernation Spray	400	30	30s	The targets in the cone will fall into a deep slumber for up to <dur> seconds. Empowering causes an uneasy sleep, reducing magicka and stamina regeneration by <mag>%.
Night Vision	337	-	300s	Dark Sight for minutes.
Nightmare	300	10	30s	The touched faces his worst nightmare, dealing <mag>% less damage for up to <dur> seconds. Empowering the spell can kill sleeping targets instantly.
Phantasmal Pain	300	40 (10)	-	The touched target believes to have suffered a grievous wound, reducing health by <mag> for up to <dur> seconds. If empowered, the severe pain also reduces movement speed by <mag>%.
Shadow Shield	150	54 (10)	86400s	The caster shifts his body partially out of this plane, raising his armor rating by <mag>. Empowering the spell grants immunity to most impact effects and increases his attack speed by (mag)%.	Upkeep: 5
Shadow Stride	100	48 (40)	86400s	The caster becomes invisible until he interacts with the environment. The spell also increases armor rating by <mag> and reduces movement noises by (mag)%. Sprinting will break the required concentration for this spell. If the spell is not empowered, running will also break it.	Upkeep: 8
Soothe (Rank III)	236	30	30s	Exposure is reduced by <mag> for seconds. When the effect wears off, the exposure is gained again.

Spells	Base Cost	Magnitude	Duration	Effect	Notes
Blackout	800	10 (50)	24s (4s)	The caster evokes an orb that causes a mental blackout on impact, draining <mag> magicka per second and reducing the magicka regeneration by (mag)% for up to <dur> seconds. If empowered, the spell also negates the victim's every thought, causing unconsciousness for up to (dur) seconds.
Fade Other	251	-	30s	The caster renders the target entity invisible for <dur> seconds.
Fog of Shadows	750	-	120s	The caster weaves shadows and madness into a fog that lasts for <120> seconds, slowing its victims and rendering them more susceptible to illusions. If empowered, it also cripples their magicka regeneration.
Invisibility Sphere	200	10 (48)	86400s	The caster creates a <mag> foot wide sphere which renders nearby allies and himself invisible until they interact with the environment. All inside the sphere also gain <mag> armor rating and they cannot be aimed at precisely. If not empowered, swift movement will break the spell.	Upkeep: 5
Muffling Sphere	200	40 (11.5)	86400s	The caster creates a sphere which dampens all sound caused by movement by <mag>%. The sphere has a radius of (mag)ft. If not empowered, the spell breaks if the caster attempts to move swiftly.	Upkeep: 3
Phantasmal Killer	900	-	3s	The caster commands a weak-minded target to commit suicide but slicing their wrist. Empowered, the spell's message becomes even darker and universal, causing an immediate heart attack to most living creatures.
Shadow Simulacrum	300	-	86400s	The caster calls upon the power of shadow magic, weaving darkness into a semi-real illusion of a dremora. Those who realize the deception suffer less grievous wounds from him. Empowering the spell allows the shadow weave to quickly restore itself after being severed.	Upkeep: 7
Veil of Silence	200	40	86400s	The caster enshrouds himself in a veil of silencing shadows that absorbs whatever sound is caused by his illusion spells and <mag>% of the noises caused by his movement. If not empowered, the caster cannot maintain the spell while moving swiftly.	Upkeep: 6

Spells	Base Cost	Magnitude	Duration	Effect	Notes
Mind Maelstrom	1100	-	20	The caster unleashes an orb which will release a huge mind maelstrom upon impact. Its victims may be affected randomly by fear or charm effects. If empowered, the victims can also be affected by frenzy or sleep.
Mistress of the Dark	500	-	86400s	The caster calls upon the power of shadow magic, weaving darkness into a semi-real illusion of a dremora countess. Those who realize the deception suffer less grievous wounds from her. Empowering the spell will make her move like lightning.	Upkeep: 5
Phantasmal Miasma	300	10 (40)	86400s	The caster creates a <mag> foot wide sphere of doom and suffering, causing illusions of painful injuries. Targets who do not resist the spell have their health reduced by <mag> as long they stay within the bounds of the aura. If empowered, the severe pain also reduces movement speed by (mag)%.	Upkeep: 5
Shadow Sancturary	500	40	86400s	The caster shifts his body out of this plane, becoming invisible and invulnerable.It also reduces his movement noises by <mag>% and negates all his damage done. If the spell is not empowered, running or sprinting will break it.	Upkeep: 6
Veil of Shadows	400	11 (40) [48]	86400s	The caster creates a <mag> foot wide sphere which enshrouds nearby allies and himself, making them invisible until they interact with the environment. The sphere also reduces the noise caused by movement by (mag)% and grants <mag> armor rating. If not empowered, the caster cannot maintain the spell while moving swiftly.	Upkeep: 4