Soundtrack composed by Frank Klepacki
Here is another track that features when you play as the Hierarchy. "Doom of the Aliens" is one of the more memorable ones and one that I can immediately associate with the Hierarchy.
Speaking of the Hierarchy, why exactly are they bringing doom to us humans anyway? Well, the Hierarchy knows nothing but conquest and exterminating other species. When they arrive at Earth, their goal is to strip mine the planet of resources, including organic ones. This is your goal if you play as the Hierarchy but there are two other factions that will aim to stop you: the robotic Novus and the enigmatic Masari. Oh, there's also the Humans I suppose but we don't really put up a fight in the end.
The soundtrack was originally offered for free off the Petroglyph forums but you can download a copy from the Free VGM section at Choicest Games.