Unique Attack Modifiers (formerly known as Orb Effects) are special 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, Broodmother's Incapacitating Bite, and Ursa's Fury Swipes. Not all attack modifiers count as Unique Attack Modifiers.
Multiple modifiers do not stack; for each attack one modifier always takes priority. The order of priority is as follows:
- 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)
- Ability Modifiers - Modifiers granted by Hero abilities*
- Standard Modifiers - Modifiers granted by items, with the oldest one in a Hero's inventory taking priority (priority can be shifted by dropping an item and picking it up again)
* 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.
There are three known exceptions: Eye of Skadi or Orb of Venom, and Lifesteal can be active at the same time. (Lifesteal is granted by Morbid Mask, Helm of the Dominator, Mask of Madness, or Satanic.) Eye of Skadi will also stack with Clinkz Searing Arrows, though only if autocast (not manually cast).
 List of Items and Abilities that Grant Unique Attack Modifiers
 Orb walk
Typically when you right-click on an enemy hero, the nearby (500 radius) creeps will agro you, and attack you. When you cast spells on enemy heroes this is not the case.
Orb-Walking involves using abilities that have Unique Attack Modifiers. Select the ability once and click on an enemy hero, this is now considered casting an ability; nearby enemy creeps will not attack you. You can repeat this as many times as necessary. Right clicking the ground to move in between casts of your Unique Attack Modifier is where the 'walk' comes in, and allows you to land more attacks on a fleeing enemy.
Conversely, if you enable auto-cast for the Unique Attack Modifier and right-click on a hero, creeps will attack you.