Help creating a spell

[Twilight]

I want to create a spell called :Horns Call (Gives bonus armor to nearby ally units for 4/6/8 seconds by 5/10/15 armor). I thought of using the Scroll of Protection as a hero ability, but not sure if it works.

 

[Jampion]

Try it out yourself. Problem could be, that the ability has no casting animation.
I think roar/howl also have armor fields.

 

[Abovegame]

As Jampion said: