@GIMLI_2 & aple
I dont think that he wants an aoe spell as object editor ability.
To actually answer the question.
1, You have to read this entire to-do list.
2, You have to make a dummy unit.
3, You have to move the dummy unit every 0.03-0.05 seconds in the direction it should be moving to.
4, You have to calculate when it should end travelling.
5, You have to make the dummy unit visually noticeable by adding an attachment to it for example.
6, You have to index the missiles and store the casters, targets, speeds, directions, models, collisions, etc.
7, You have to have an event that fires when you want to apply the effect of your spell (damage, heal, crowd control, whatever)
8, You have to realize that you have to do a lot more than this.
9, You have to go to this website and search for a system that does all the above (except 1) for you without you having to care about any of them. These are called missile systems.
10, You have to thank the one who made that system for having made such a nice resource that does exactly what you want it to do.
11, You give him +rep.