I thought I would post this class first as it is relatively simple to implement and can be put into any XNA engine and is not specific to the HM Engine.
Right, the effect of this class can be seen in the earlier post I gave for fog. So without further a do, here is the class:
So all you have to do is populate the Fog class static properties with the required values and in your assemblies Draw method call the static Draw method, like this:
And with that you can have a nice fog effect in your engine.