Here are some issues:
1) You don't need LightningArrow2, that trigger is unnecessary. Everything can be done inside a single trigger.
2) (Target point of ability being cast) doesn't exist, since your ability doesn't target a point, it targets a unit.
3) Regarding LightningArrow2, which you can delete, it has issues. First, you don't want to use Entering Unit and Waits like that. The only safe Event Response to use after a Wait is
Triggering Unit. But that's unnecessary, you can simply add an Expiration Timer to your Dummy unit in the same trigger that creates it like I do below. Also, Learn Skill is used for Heroes, to increase the Level of their Hero abilities. You want to use Add Ability/Increase Level of Ability when modifying the abilities for a Dummy unit. Your Dummy unit should be based on the Locust unit (You should use the one I made in my map, it's setup properly).
The fix:
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Events
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Unit - A unit Starts the effect of an ability
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Conditions
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(Unit-type of (Triggering unit)) Equal to Nhamun
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(Ability being cast) Equal to Lighting Arrow
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Actions
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Set Variable TempPoint = Position of (Target unit of ability being cast)
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Unit - Create 1 DummyLightingArrow for Triggering Player at TempPoint facing Default building facing degrees
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Custom script: call RemoveLocation (udg_TempPoint)
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Unit - Add a 10.00 second Generic expiration timer to (Last created unit)
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Unit - Add Chain Lightning to (Last created unit)
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Unit - Set level of Chain Lightning for (Last created unit) to (Level of (Ability being cast) for (Triggering unit))
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Unit - Order (Last created unit) to Orc Far Seer - Chain Lightning (Target unit of ability being cast)
Some random notes:
- Since you want to use this method, the unique Dummy unit becomes unnecessary and you can use a generic Dummy unit instead.
- It's spelled Lightning with an "n"
Also, I'm not sure what was wrong with the spell I made, but it's very customizable. You could trigger any damage/effects you could possibly want. The Lightning Arrow (Hero) ability could be based on Storm Bolt if you wanted it to stun/damage the first target for example. You could also trigger some "Impact" damage yourself if you wanted the primary target to take extra damage (look up Cause Unit Damage action).