Blackening a circular area of terrain

So in a spell I'm experimenting with, I'd like to darken a 500 radius patch of ground that a spell is burning units in. I've looked at how the Shadow Dagger from DotA darkens a trail of ground, and I see that invisible units with only a shadow are used to incrementally darken the ground in a line. However, my spell, as I said, occupies a somewhat large circle. I want to minimize the number of units I create, so I don't think creating something like 20 units to encompass the circle would be efficient. I found a circular shadow model, and using that would require 20 sustained special effects to make the circle (but it is quite dark, I like the look). I tried using a massive 1000 wide/high flying shadow for only one unit, but because of its size the direction of the in-game sun moves it far away. What do you guys think is the best solution? Is there a 1-unit solution that has a centered circle?

compensate for the offset of the flying units shadow?

Sounds like a plan, but I have no idea what values to put into the offset fields, I assume they are proportionate to the height and width of the shadow....

Yep, offset x and y = 1/2 width and height. + rep, if I'm allowed to rep you more.

:( I gotta spread more around, we'll see in the future.