Unique Attack Modifier

Unique Attack Modifiers (formerly known as Orb Effects) are special attack modifiers that alter a Hero's basic attack to provide an additional effect. An example of this is Morbid Mask, which grants the lifesteal effect. Certain Hero abilities are Unique Attack Modifiers, such as Anti Mage's Mana Break. Not all attack modifiers count as Unique Attack Modifiers, such as Huskar's Burning Spear or the purchasable Diffusal Blade. This means that Heroes can combine a unique attack modifier with other non-unique attack modifiers like Burning Spear or Searing Arrows, making use of both at the same time

Priority
Multiple modifiers do not stack; for each attack one modifier always takes priority. The order of priority is as follows:


 * 1) Conditional Modifiers - Modifiers that only occur on some attacks and are not controlled by the player, such as Maelstrom (chance-based) and Mjollnir (chance-based). If a Conditional Modifier fails to proc then a Standard or Ability Modifier will be present on the attack (if applicable)
 * 2) Ability Modifiers - Modifiers granted by Hero abilities* (in Ability Draft the first UAM ability obtained will override later ones)
 * 3) Standard Modifiers - Modifiers granted by items (an item will take priority depending on which is the oldest in your inventory, or in the case of illusions the first slot in reading order)

* ''UAMs from hero abilities, such as Viper's Poison Attack, will only be overridden by conditional modifiers if they are set to Autocast. If they are Manually cast, they cannot be overridden.''

When attacking units (such as Spell Immune heroes or buildings) that are immune to certain Unique Attack Modifiers, other UAMs that can proc will be used, even if they are not prioritized. An example of this is having an Eye of Skadi and a Desolator, if the Skadi has higher priority. Because buildings are not affected by the Skadi, the Desolator's effects will be used instead.

There is one exception: Eye of Skadi and Lifesteal can be active at the same time. (Lifesteal is granted by Morbid Mask, Helm of the Dominator, Mask of Madness, or Satanic.)

Sources of Unique Attack Modifier
Abilities


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Items


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Harassment
Issuing an attack command on an enemy hero will cause nearby (500 radius) creeps and towers to switch their aggro to the attacking unit. When casting spells on enemy heroes, this is not the case. For heroes with active UAMs, manually casting the ability causes the attack to count as a spell; this allows one to attack enemy heroes without gaining creep aggression. The opposing hero must either take damage from creeps when returning fire, kite creeps and so deal less damage, or not retaliate at all - this gives heroes with orbs a strong presence in the laning stage.

Conversely, if you enable auto-cast for the Unique Attack Modifier and right-click on a hero, creeps will attack you.

Orb Walking
In DotA Allstars (exclusively), manually casting UAMs used a hero's cast animation time rather than attack animation. As this was usually faster at early levels, manually casting orbs allowed the projectile to be launched more quickly. In conjunction with animation cancelling, this allowed a chasing or kiting hero to constantly deal damage while losing very little ground; if the orb in question had a slowing effect, an enemy hero with no escape could often be kited to death. Taking advantage of this was known as Orb Walking. This is no longer the case in DotA 2, only the aggro effect remains.

As the animation effect is no longer present in DotA 2, the term 'Orb Walking' is now often used interchangeably with using orb effects to harass without gaining creep/tower aggro.