Buff check trigger

I've been looking around for awhile now and simply can not figure out a way to do what I want to do.

What I'm trying to do with a trigger is firstly

Event -> Hero casts ability

Condition -> Ability = to Ability X and Buff = to Buff Y

Now for the actions what I want to do is first make an action to check the buff applied is equal to say Buff Y then have the casting unit (triggering unit) damage all units effected with Buff Y for X damage.

I hope that was clear enough >> Actually let me just copy and paste this >>

"Causes any of the Death Knight's diseases on the target, and all enemies within 30 yards of the target, to painfully erupt, dealing 77 to 93 Shadow damage."

Basically I want to create an ability like this.

What buff actually? You think there is a triggering buff event response? there isn't.

Your tooltip explains what the spell does better than all that text you wrote.

Basically, the death knight has several buff spells, and this one area-of-effect spell that deals damage for each of his buff spells on the affected unit(s).

What you need to do is pick all units in the cast area and for each of them, check which of the buffs it has and for each of those, deal damage.

Boolean comparison > Unit has Buff?

Well I treated this situation in my tutorial (check my signature), when you use the "start spell effect" with missile spells, it will trigger it when the missile is launched but the buff will not be present, therefore doing this buff check will return false. I solve this with a periodic timer that checks the buff presence, if so, it will start working.