Spawn gameObject horde, modify concentration of spawned objects.

How would I create a non-uniform random generation of gameObjects (enemies) that mimic the formation of a horde like this image:
79008-horde.png

I want there to be more gameObjects in the front and less as it trails off towards the back. I thought of making an Empty gameObject and having the enemies target with code like this:

public Vector3 target : Transform;
 if (target == null && GameObject.FindWithTag("Empty"))
 {
          target = GameObject.FindWithTag("Empty").transform;
 }

But that won’t give me the “trail” effect. The code for my randomly generated enemies is :

	void SpawnHorde()
	{
		for (int i = 0; i < hordeCount; i++) 
		{
			Vector3 spawnPosition = new Vector3(Random.Range (0, 200), 50, Random.Range (0, 200));
			GameObject obj = Instantiate(Resources.Load ("Prefabs/Sphere"), spawnPosition, Quaternion.identity) as GameObject;
		}
	}

Right now they are cubes but later will be replaced with a different prefab.

If anyone has any suggestions, they would be greatly appreciated! I am stumped right now

Figured it out