Several units and Heroes in Warcraft III possess the ability to cast spells. Some spells improve friendly units or weaken enemy troops. Other spells do direct damage to enemies or heal allies. All spells cost a certain amount of Mana to cast. Physical Abilities/Spells do not require mana. Many people use "spells" to cover both spells and abilities. Most high-cost spells must be manually cast. This means you must first select the spellcaster, click on the spell icon, and then left-click on a target.

Spell Categories
Spells come in three varieties: Physical, Magical, and Universal.

Physical Spells
Physical Spells cannot be dispelled, do affect magic immune units and normal units, and do not affect ethereal units.

Examples
- Poison of various types (NOT Shadow Strike)
- Ensnare & Web
- Critical Strike/Bash/other attack buffers
- Disease
- Similar spells which DO NOT require mana to cast (mana cost spells are ALWAYS magical or universal)

Magical Spells
Magical spells can be dispelled, do not affect magic-immune units, and affect ethereal units and normal units. Magical spells do not affect buildings or mechanical units.

Examples
- Storm Bolt
- Carrion Swarm
- Bloodlust
- Polymorph

Universal Spells
Universal spells can work against anything, and cannot be dispelled. They include ultimates and healing spells. Some Universal spells cannot work with Mechanical Units.

Examples
- Heals
- Ultimates
- Auras

Duration
Duration is the length of time the spell is active.

Cooldown
Cooldown is the amount of time before you can cast the same spell again. This is also known as a Casting Delay. Cooldown is fixed and unaffected by increases to Mana capacity and Mana Regeneration. Some spells do not have a Mana cost but are instead limited by their cooldown.

Intelligence
Intelligence for Heroes determines the total Mana. Adding to Intelligence increases the Mana pool. It also slightly increases mana regeneration rate / speed.

Mana Regeneration
Mana Regeneration determines how fast Mana regenerates. Increase Mana Regeneration will return Mana faster, allowing for more spell casts within a given time period.

Researching Spells
Most spells need to be researched. Heroes must level up and spend skill points to learn new spells.

Spell Channeling
Some spells must be "maintained" to achieve their full effect. This applies mainly to mass-damage area effect spells. For example, if you were to cast an Earthquake spell on an enemy town, your Far Seer must stay casting the spell until the Earthquake is finished. If you give him a new order during the spell, the Far Seer will stop casting the Earthquake.

Spells affected: Blizzard, Earthquake, Tranquility, Starfall, Death and Decay, Big Bad Voodoo, Mass Teleport, Tornado, Stampede, Cloud, Aerial Shackles, and Rain of Fire.

You can interrupt this spell with spells such as:

Humans: Sorceress - Polymorph (uncastable on heroes)
Humans: Dragonhawk Rider - Aerial Shackles (uncastable on ground units)
Humans: Mountain King - Storm Bolt
Humans: Mountain King - Bash (uncastable on air units)
Orcs: Raider - Ensnare
Orcs: Tauren Chieftain - War Stomp (uncastable on air units)
Orcs: Witch Doctor - Stasis Trap (uncastable on air units)
Orcs: Shadow Hunter - Hex
Night Elves: Druid of the Talon - Cyclone (uncastable on air units)
Night Elves: Keeper of the Grove - Entangling Roots (uncastable on air units)
Undead: Crypt Fiend - Web (uncastable on ground units)
Undead: Dread Lord - Sleep
Undead: Dread Lord - Inferno (uncastable on air units)
Undead: Crypt Lord - Impale (uncastable on air units)
Neutral: Dark Ranger - Silence
Neutral: Dark Ranger - Charm (uncastable on heroes)
Neutral: Goblin Tinker - Cluster Rockets
Neutral: Pit Lord - Doom (uncastable on heroes)
Neutral: Fire Lord - Soul Burn
Neutral: Fire Lord - Volcano

In order to ease your micro-management, giving an order to the whole group doesn't affect the Hero that is busy with his Spell Channeling. This means that if you also want to interrupt the Hero from casting, to retreat or attack a specific target, you have to give him the order again when selecting him only (or using a sub-group order).

Faerie Dragon - Mana Flare - this must be maintained like with other spell channeling spells but it cannot be stopped by an enemy unless they kill the Faerie Dragon itself.

Buff
A Buff is a beneficial spell. An example of a "buff" is the Priest's Inner Fire.

Dispel or De-Buff
You can remove hostile spells from your troops or Buffs on enemy units using a Dispel type spell:

Humans: Priest - Dispel Magic, Spell Breaker - Spell Steal
Orcs: Shaman - Purge, Spirit Walker - Disenchantment
Night Elves: Dryad - Abolish Magic, Wisp - Detonate
Undead: Banshee - Anti-magic Shell, Destroyer - Devour Magic
Neutral: Storm - Dispel Magic

Dispel cannot remove every spell or get rid of area effect spells such as Blizzards. Typically Dispel will simply remove buffs or de-buffs on units. Spells that can be removed are buffs and curses such as Bloodlust, Curse, Cripple, Faerie Fire, Slow, Inner Fire, and so on. Some effects cannot be dispelled, such as Poison and the slowed movement from Purge.

Most Dispel abilities can also damage hostile summoned units, illusions, and wards. Dispel does not instantly kill or remove summoned units as some might expect. Each Dispel ability has specific amount of damage it does to summoned units. Summoned Units will only be killed instantly if you do enough damage to them to reduce their hit points to 0. Unlike other Dispel abilities, Anti-Magic Shell does not damage summoned units. Only enemy or hostile Summoned Units are damaged by Dispel. Summoned units that have Spell Immunity (like Infernals) can not be damaged by dispel effects.

Abolish magic automatically removes positive buffs from enemy units, and negative buffs from friendly units when in autocast mode. If you use the spell manually it will remove all buffs. Spell Breaker's Spell Steal removes positive buffs from enemy and negative buffs from friendly units. All other Dispels remove all buffs from their targets.

Autocast Spells

Some spells and abilities do not need to be manually activated by the player. They are called Autocast spells, and you can set them to be automatically cast by the unit when needed. To toggle a spell or ability on or off, simply right-click on the spell icon and the borders of the spell will highlight, indicating that it will now be Autocast by the computer. When the unit encounters a situation that would call for the casting of the Autocast spell, that unit will automatically cast it. For example, a Human Priest that sees a wounded friendly unit will Autocast Heal to cure the damaged unit, while an Orc Shaman will automatically cast Bloodlust on a friendly unit once that unit begins attacking an enemy target.

Autocast Spells or Switch Spells are spells that have toggles. The Human Priest for example has Healing as an Autocast Spell. When it is toggled on, the Priest will heal any damaged friendly unit in range as long as he has Mana. If toggled off, the Priest will not heal damaged units in range. Casters with Autocast Spells have at least two different spells you can switch between. The Priest also casts Inner Fire. A group of Priests could be casting a mix of Healing or Inner Fire.

All positive buff auto-cast spells prefer the closest friendly unit. If there is no friendly unit near the caster, they will cast it on themselves, but only if the normal Autocast rules apply. For instance, a solitary Shaman will not Autocast Bloodlust on himself until he attacks someone. The same is the case for Priests and Inner Fire.

How do you decide which spells are Autocast?
1) The spell must have an easy and consistent rule that we can apply for the casting AI, so the player can easily predict how his casters will act.
2) High Mana cost spells are not made Autocast since they require a higher level of decision making to cast. It is easy to say all enemy units should be slowed, but we do not want to decide which unit should be polymorphed.
3) Spells that would be menial to cast are generally made Autocast. For example, in Warcraft II the player had to manually cast heal and bloodlust, despite the fact that the player would want to cast these spells in every battle if they had the click reflexes to do so.

If you use the same Aura from two Heroes they do not stack, but since the Auras pulse on and off, the units affected will be covered by the Aura more often.

Heroes with Auras benefit from their own Aura.

Auras are Universal spells. Universal spells, unlike Magical spells, can work against anything. Magic-immune units like Dryads, are affected by Auras from friendly units only. Dryads will receive the Trueshot Aura from a friendly Priestess of the Moon and be unaffected by Thorns Aura from an enemy Keeper of the Grove.

Auras will not be active if the unit is Hidden (Shadowmeld) or Invisible.

Spellcasters
Each race has 2 dedicated spellcasters. They all start with 200 Mana and gain a total of 400 Mana with Adept and Master Training. Spellcasters are typically weak and need protection.

Humans: Priest, Sorceress
Orcs: Shaman, Witch Doctor, Spirit Walker (different stats)
Night Elves: Druid of the Claw, Druid of the Talon
Undead: Necromancer, Banshee

It's best to build two Spellcaster buildings (such as two Arcane Sanctums) so that you can both train new units and research upgrades at the same time. This also allows you to train two different types of spellcasters at once or to train several identical units more quickly. For example, with two buildings you can build a Sorceress and a Priest at the same time.

The Night Elf Druids both have another animal form which changes them into different types of units.

Anti-Spellcasters
Anti-Spellcasters are, of course, designed to fight spellcasters.

Humans: Spell Breaker
Orcs: Spirit Walker
Night Elves: Dryad, Faerie Dragon
Undead: Destroyer

The Dryad is sort of a hybrid unit since it is both a spellcaster and a combat unit.

Dark Ranger: Silence
The Dark Ranger's Silence ability can be used to prevent spellcasters from casting spells.

Spell Research
Spellcasters must research Adept Training and Master Training before they receive all of their spells. These training upgrades give the spellcasters a total of 200 extra Mana and increase their Mana regeneration rate. These upgrades are worth getting because of the extra Mana and Mana regeneration. For example, Priests receives Healing without training, but the additional training will allow them to Heal more often.

Researching Master-level spells (Bloodlust, Inner Fire, etc.) requires a 3rd-level Town Hall (Castle, Fortress, etc.).

Line damage spells
Line damage spells such as Shockwave and Carrion Swarm do not damage a targeted friendly unit.

Ultimate Spells
Each Hero has one Ultimate Spell -- his most powerful. To use this spell a Hero must be at level 6 or higher.

Channeling ultimates cancels invulnerability. If an invulnerability spell is used while channeling, channeling will end.

Ethereal
Ethereal means: "Characterized by lightness and insubstantiality; intangible." Units that are Ethereal can only be damaged by units that have the attack type "magic" such as spellcasters. Ethereal units cannot attack but can cast spells. Ethereal units take 66% bonus damage from spells and magic damage type.

Units healed while Ethereal gain an additional 66% benefit. For example, casting level 1 Holy Light on an Ethereal unit for 200 hit points actually ends up healing 333 hit points. Only "spell" healing is affected by this; item healing and aura healing are not.

Spell Immunity
Some Creeps and units such as Golems, Dryads, Infernals, and more are Immune to magic. Spell Immune units cannot receive positive buffs such as Roar, Bloodlust, or Inner Fire nor receive negative de-buffs such as Curse or Slow. They will receive Auras since auras are non magical. They also cannot be damaged by units who have the "magic damage" attack type. Spell Immune units can be hit by certain abilities.

Resistant Skin
Reduces the duration of negative spells and renders the unit immune to certain spells. Resistant Skin means you can't charm it and use abilities like Entangling Roots and Ensnare. Other abilities are made less powerful.

Items
-Amulet of Spell Shield - Blocks a negative spell that an enemy casts on the Hero once every 40 seconds.
-Runed Bracers - Reduces magical damage dealt to the Hero by 33%.

You Cannot Dispel Summoned Units
If this were allowed, summoned units would be too easily killed and would lose their purpose. Players would not use summoned units for fear of their being dispelled. It would be much easier to get rid of them than to create them.

Most Dispel will damage summoned units and illusions.

Passive Spells and Abilities
Some units possess abilities that do not need to be cast or even turned on. These abilities are called passive abilities, and operate as soon as the upgrade is researched. The Wind Rider's Envenomed Spears are one example. Once you research this upgrade at the Beastiary, the attacks of all your Wind Riders will automatically gain the poison ability from the Envenomed Spears. Once you research a passive ability, you will see it noted by a button in the unit's command card, even though you do not need to press it to activate the ability. It is always on.

Poison Spells
Poison will not kill units, instead will leave them with two hit points.

The Undead's Disease Cloud also does not kill units and cannot be dispelled.

Poison stacks, though only from differing units. Example: If 1 unit is attacking a target with poison that does 5 damage a second, the unit takes 5 damage a second from poison as long as it is poisoned. Multiple attacks from the same unit do not increase this. If 2 units of different types attack a target, and they each possess a poison attack that gives 5 damage per second, the target takes 10 damage per second.

Area of Effect Spells (AOE)
Damage-capped AOE spells have a seperate cap for friendly, enemy, and neutral units.

Reincarnation Ability
You do not get any experience if you kill a unit that triggers the Reincarnation ability. If you kill one of these units "for good" you do get experience, however.