make dark fire

How to make dark fire that appear after pressing a key then disappear after making the order

You could do this:

var darkfire : ParticleEmitter;//Or GameObject, or whatever!

function Update(){
if(Input.GetKeyDown("f")){
   if(darkfire.gameObject.active == false;){
      darkfire.gameObject.active = true;
   }
if(darkfire.gameObject.active == true){
   darkfire.gameObject.active = false;
}
}
}

This is just the main code, add some if’s or some cases to it to make it more advanced
If you’re using legacy particles replace darkfire.gameObject to

darkfire.emit = true;

or

darkfire.Emit();