Some old buddies from Might and Magic 2 return! Sadly they're much less fun this time.
I forgot to do it on-camera but you can earn membership and buy Light/Dark spells at Mire of the Damned. You can also become an Expert in Light there, but to attain Mastery you'll have to do two things.
1. Have Saintly reputation (1000 or more) and at least one Cleric or Sorcerer.
2. Take the south dock from Silver Cove on a Wednesday and sail to the "North Island". This will take you to the Eel-Infested Waters. There you can train with Ki Lo Nee on the island at the center-bottom of the map to get mastery.
Light and Dark are far and away the two most powerful schools of magic, so the sooner you can get em and start funneling points in, the better!
The Master trainer for Dark magic is in Paradise Valley. It is possible to get there now (and quickly rack up the requisite negative Karma with a single cast of Armageddon), but if you're going to do it, I strongly suggest you avoid combat as much as possible - the Titans there are insanely dangerous and will stomp you flat if you try to fight them head on (at least until you get 150 levels or so under your belt).