Easiest way to make a 100% custom aura?

I want to make a slow aura, but I can't use unholy aura cause I want to manually (with code) slow the unit. How can I detect whether a unit is or is not anymore effected by an aura?
So.., I kinda want to detect all units coming within a certain range of the auraunit, but I also want to detect when the units come outside of that certain range of the auraunit.

You may check for aura buff

Use the ability Tornado Slow, its a slow aura, it should work great =).

Vexorian's aura template.

It's typically preferable to use an ability to slow rather than reducing movement speed by triggers because other factors could cause permanent changes in movement speed.

Nah I NEED to use triggers.- so I guess Vex's aura template, or doing something with checking whether the unit has an aura buff, are my options.