Spell based on forked lightning (how to affect multiple targets)

[walzuu]

I'm trying to create a spell based on forked lightning which causes all targets to get a doom-like buff. I tried

Event:
Unit - A unit owned by Player 1 (Red) Begins casting an ability

Condition:
(Ability being cast) Equal to Fel Blood Knives

Action:
Unit - Add a 5.00 second Generic expiration timer to (Target unit of ability being cast)

Now this only affects the single unit I target and ignores all the rest. I couldn't find any reasonable solution since there doesn't seem to be any "source of damage equal to that specific skill" conditions or anything like that.