The Chain Lightning base damage must be 0.01 or 0.1 or some non-zero number for the spell to work properly, especially for AI casting.
As for the trigger, I'd assume that when ever a unit is targeted by an ability (event) equal to Chain Lightning (condition) apply Orb-of-Corruption-bonus? to triggering unit(action) should work {at least point you in the right direction. I'm not sure if the2nd, 3rd, 4th, etc.. units are targeted though. Also not sure about the units between targeted units though...
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