The School of Illusion involves manipulating the mind of the enemy. This skill makes it easier to cast spells like Fear, Calm and Invisibility.

The Illusion tree is good for both defensive and offensive mages that want to add some crowd control. Illusion also can prove useful for Stealthy characters such as thieves ans assassins because of spells like Invisibility and Quiet Casting. If you are creating a sneaky character, consider to use Illusion spells and take some perks from this tree.

Level up the Illusion skill by casting Illusion spells effectively.

TRAINERS

Expert Trainer: Atub (The Rift)
Master Trainer: Drevis Neloren (College of Winterhold)

SPELLS

Novice:
Courage: Target won't flee for 60 seconds and gets some extra health and stamina
Clairvoyance: Shows the path to the current goal
Fury: Creatures and people up to level 6 will attack anything nearby for 30 seconds

Note: Clairvoyance is a very useful spell to have for ANY character, even if you don't use magic at all. It helps guide you to the next goal so you do not get lost.

Apprentice:
Calm: Creatures and people up to level 9 won't fight for 30 seconds.
Fear: Creatures and people up to level 9 flea from combat for 30 seconds
Muffle: You move more quietly for 180 seconds

Adept:
Frenzy: Creatures and people up to level  14 will attack anyone nearby for 60 seconds 
Rally: Targets won't flee  for 60 seconds and get some extra health and stamina

Expert:
Invisibility: Caster is Invisible for 30 seconds. Activating an object or attacking will break the spell
Pacify: Creatures and people up to level 20 won't fight for 60 seconds
Rout: Creatures and people up to level 20 flee from combat for 30 seconds

Master:
Call to Arms: Targets have improved combat skills, health, and stamina for 10 minutes
Harmony: Creatures and people up to level 25 nearby won't fight for 60 seconds
Hysteria: Creatures and people up to level 25 flee from combat for 60 seconds
Mayhem: Creatures and people up to level 25 attack anyone nearby for 60 seconds

PERKS

Novice Illusion
Cast Novice level Illusion spells for half Magicka

Apprentice Illusion
Cast Apprentice level Illusion spells for half Magicka
Requires: Novice Illusion, Illusion 25

Adept Illusion
Cast Adept level Illusion spells for Half Magicka
Requires: Apprentice Illusion, Illusion: 50

Expert Illusion
Cast Expert level Illusion spells for Half Magicka
Requires: Adept Illusion, Illusion: 75

Master Illusion
Cast Master Level Illusion spells for Half Magicka
Requires: Expert Illusion, Illusion 100

Note: Novice, Apprentice, Adept, Expert and Master Illusion Perks are ESSENTIAL for any Illusion mage, or character who plans to use illution a lot.

Illusion Dual Casting
Dual Casting an Illusion Spell overcharges the effects into an even more powerful version
Requires: Novice Illusion, Illusion 20

Note: Useful to take to increase level cap of targets you can affect

Hypnotic Gaze
Calm spells now work on higher level opponents. Cumulative with Kindred Mage and Animage
Requires: Novice Illusion, Illusion 30

Aspect of Terror
Fear spells work on higher level opponents. Cumulative with Kindred Mage and Animage.
Requires: Hypnotic Gaze, Illusion 50

Rage
Frenzy spells work on higher level opponents. Cumulative with Kindred Mage and Animage
Requires: Aspect of Terror, Illusion 70

Animage
Illusion spells now work on higher level animals
Requires: Novice Illusion, Illusion 20

Kindred Mage
All Illusion spells work on higher level people
Requires: Animage, Illusion 40

Quiet Casting
All spells you cast from any school of magic are silent to others
Requires: Kindred Mage, Illusion 50

Master of the Mind
Illusion spells work on Undead, Daedra, and Automatons
Requires: Quiet Casting or Rage, Illusion 90