Nojix... targ is a unit variable, and you assign it the (Target Unit of Ability Being Cast) at the beginning of the trigger. Read the FAQ to understand.
And now, you want it to cast a chain lightning spell, and from each target hit by the chain lightning to create a fork lightning at a random unit? I'm afraid that's impossible, unless you trigger the chain lightning yourself. Chain Lightning is special. You can't determine the targets hit by it. You can do it with single target spells though. So be more specific about what you want, and about what the spell is supposed to do.
~Daelin