Burning arsenal spell, need help

Ok so im trying to make this ability (aura) called burning arsenal, and it works like this. Any unit under its effect gets a seperate ability (coustomized poison sting) which deals damage over time. However i cant find the trigger that gives you the ability when your under the abilities effects AND removes it when your not, so far i can add it but its perminant, please i need an aura detection trigger. Just so you know, i know how to give the target units the ability, i just dont know how to remove it.

Many thx!!

I think it's Unit - 'Add Ability' and 'Unit - Remove Ability'

Ok so maybe that wasnt too clear. Im after something like this.

Events
Unit - Aunit comes under the effect of an aura

If unit is still under auras effect the add ability Burning Arsenal to target unit of ability being cast else do nothing.

That obviously doesnt exist but i was wondering if there is like an alternative to that?

Ta

Weird, this trigger has stumped me so far... :(

For the event I used "Unit - Enters within (900) of (whatever hero)

Then for the rest I am stumped...

This should be what you want:
event-every 1.00 sec. on the game
Unit group pick every unit in playble map area matching level of (Your Aura) for matching unit is greater than or equal than 1
Loop-Actions set variable to the picked unit.
-Unit group pick every unit in within (range of the aura) of (unit picked before) matchinhg owner of matching unit is an ally of owner of ( unit picked before)and do action-unit add abillity burning arsenal to picked unit.
For removing the abillity
events-every 1.50 sec on the game
Unit group pick every unit in playble map area matching level of burning arsenal for matching unit is greater than or equal than 1 then do- remove abillity burning arsenal for picked unit.
I am not sure if this will work right, you will have to test it, but I am sure that it will cause some lag because of the timmed events, but if no one else says anithing.

Ok, i would just give the aura based off of an ability like phonix fire (i think that does damage if ur within range of the unit, but mayb not) but for the trigger, for where u said there isnt like if unit has effect or whatever, there actually is. Its a boolean - unit had buff

((Triggering unit) has buff Big Bad Voodoo) Equal to True

u could use that. Im still not realy sure exactally what u want, but i hope this helps.