Just though I share what an awesome theme Artorias has. As always, the themes tend to be more of an "Epic" scale than fast-paced OST; and that is how I like them...
The song has 2 things:
1) The mixture of all the sounds in between the voices and the instruments make an "epic" feeling, which I associate with fighting a great foe. They didn't try to go with a fast-pace theme (not that they much) to try to make it sound like you're going for a big fight, but rather the slow, but strong sounds.
2) They seem to have that "feeling" of sadness; perhaps how Artorias fell to the dark. Maybe its just me, but their is that rather sad thing about the song. Maybe it has to do with how it is slow, yet powerful.