The Source is some kind of Magic in Rivellon. You can learn a bit more about it by playing DD.
In fact, The Source was - as far as I can remember it - originally envisioned as some kind of well located under the town of the healers (Aleroth) there - or the magic was flowing out from that well, I don't remember it anymore.
Further than this I cannot remember any deeper explanation.
My mind is a bit fuzzy but I do remember Aleroth-the-village and its well, and then didn't Aleroth-the-city in Ego Draconis also have it? It had Source Square anyway, which sounds like it would be the site of the well and I'm sure there was some reference to it but I forget now.
Raze will know since he seems to indelibly remember all these factlets that the rest of us forget!
