Use "start the effect of an ability" rather than this event, mostly because this one probably won't work as it is intended for channeled spells.
For your actions, you need to create a dummy caster for the owner of triggering unit at the position of your casting unit, give it a short expiration timer (2-3 sec), add the chain lightning ability to it and order it to cast the spell on the target unit of ability being cast.
You'd also want to create a temp variable to store the point where the dummy is created, so you can safely remove it at the end of your trigger to clean memory leaks.
you can find the action under "Unit - issue order targeting a unit"
Chain Lightning AutoCast
Events
Unit - A unit starts casting an ability
Conditions
(Ability being cast) equal to "your modified slow"
Actions
Unit - Order (Triggering unit) to Orc-Farseer - 'Chainlightning' (Target unit of ability being cast)
I did that trigger string and all it does is cast the modified slow ability there is no secondary ability cast.
This trigger won't work because it assumes that your caster has both slow AND chain lightning.
This trigger won't work because it assumes that your caster has both slow AND chain lightning.
You need to create a dummy unit when slow is cast, then you can use "order last created unit to orc farseer - chain lightning" as said above.
U want the chainlighting on attack ?
Use the event "unit is attacked"
Condition either unit type = ur unit
Or unit has the ability
Then trigger the dummy unit to cast the spell on the attacket unit