Shouldn't this be in the Spells section?
This might be a bit complicated. You have to make a dummy unit first, with a model of an arrow (and raise its flying height to, lets say, 50), and make every other dummy characteristics (locust ability, don't display on minimap....blabla). Set it's speed to 1 (to avoid nasty bugs).
Now, the triggering part. You will move the unit with triggers, move it a bit a time (0.035 periodic trigger) towards the angle between the caster and the target point. Think of a way to check how much it has moved so far, and stop it when it reaches the desired distance (1500 in this case). Now, you will have to have a trigger that has a Unit In Range event, that checks if any unit is close to the arrow, if it is, kill the arrow and do something to the enemy (damage/stun).
Maybe you should have an if/then/else in the trigger which moves the arrow that checks if the arrow is dead (if it is, that means it hit someone) or maybe think of another way to check if the arrow has hit someone.
Final words: if you didn't understand a thing I said, I suggest that you learn how to trigger better. And it also means that asking someone to code a spell for you is not such a good idea.