An Aura Spell is an ability that (normally) deploys a circular region that affects units surrounding the caster.

In this tutorial, the Aura will only affect enemies, dealing them damage over time as long as they are within the range of the aura.

Overview

This tutorial will teach you how to make a negative aura (effects enemies). Specifically, we are going to create an aura that causes enemies to have their HP regeneration reduced by 1 per second. By the end of this tutorial, we'll have created the following amount of items in the Data Editor:

Creating the First Behavior (Degen Aura Periodic Searcher)

This is the Behavior that does the searching for the buff. (This will make sense in a bit, I promise.) To do this, create a new Behavior as a Buff and name it DegenAuraSearcher.

NOTE: You should consider editing the Editor - Suffix field to make this easier to deal with later.

Edit the Alignment field to Positive. This is going to create a buff on the source unit from which the aura radiates.
Take note of the Effect - Periodic field. We'll be coming back here shortly!
Scroll down to Icon and edit it to something acceptable for you.
Continue scrolling down until you reach Period. This is how often the Behavior is going to search for acceptable targets. Enter something small, such as "0.2500".

Creating the First Effect (Degen Aura Search)

This effect controls the size of our aura, the amount of units that can be effected by it, and which units we want to target. To do this, create a new Effect as Search Area.

Set Areas - Arc to 360.
Take note of Areas - Effect. We'll be back here shortly!
Set Areas - Maximum Count to "-1" for unlimited targets.
Set Areas - Radius to something. For reference, a Marine has a range of 6.
Set Impact Location - Value to "Caster Unit".
Scroll down to Search Filters and check off the box for Enemy.
Set Search Flags to "Extend by Unit Radius".

Creating the Second Effect (Degen Aura Apply Behavior)

As the sub-heading sort of suggests, this Behavior is going to have the desired effect that we want. Create a new Effect as Apply Behavior and name it DegenAura.

You're done! Sort of. We'll come back to the Behavior field in a moment.

Creating the Second Behavior (Degen Aura Debuff)

This is where the actual effect comes into play. Create a new Behavior as a Buff and name it DegenAura.

Set Alignment to "Negative". This is the buff that gets applied to Enemy units, and as such, it will show a red icon next to their wireframe.
Set the Duration to something higher than the Period we set to in Degen Aura Periodic Searcher (the First Behavior we made). NOTE: Do not set this to 0, or the effect will last forever, even if the target unit leaves the aura range.
Set the Icon to something you want.
Scroll down to Modification - Vital Regeneration Bonus and set Life to "-1". This means the units meeting the requirements of the filters will be dealt 1 damage per period.

Back-tracking and Linking it all Together

Alright, now we need to link everything together.

Go to the Buff we labelled Degen Aura Periodic Searcher (the First Behavior we made). Set Effect - Periodic to Degen Aura Search (the First Effect we made).
Go to Degen Aura Search (First Effect) and set Areas - Effect to Degen Aura Apply Behavior (the Second effect we made).
Finally, go to Degen Aura Apply Behavior (Second Effect) and set Behavior to Degen Aura Debuff (the Second Behavior we made).

Adding Degen Aura to a Unit

This part is a bit different than most other abilities. Because this aura is passive, we do not create an Ability for it. All we do is create a Button for it so it shows up in the Command Card.

Create a new Button in the Data Editor.
Name it Degen Aura.
Choose an Icon.
Add the Button to your unit by modifying its Command Card - Buttons - Ability Command. Make Command Ability as Passive and do not edit anything else.
Go to Behaviors - Behavior and set the field to Degen Aura Periodic Searcher (the very first thing we created).

Adding an Actor Graphic

To make a graphic appear on the target unit while being affected by the aura, create a new Actor of type Model and based on ModelAnimationStyleOneShot. - Set the Model to whatever you want. - Go to Events - Terms, right click and then Add Event - Use the same things shown above, except the Source Name, that you should set to your apply effect

Results!

You should get something like this:

Credit(s)

Auras! (Zantetsuken, Sc2Mod.com) Original article that this page is based on. Thanks to Zantetsuken for the work he put into writing this.
Aura Tutorial (.Talon, UDMod.com) Some additions taken from his tutorial.

Creating an Aura Spell

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