The current Shadow Magic in the game is only obtainable near the end of Khrysalis part 1. Bosses have Shadow Magic before you do. Depending on your school, you get one Shadow Spell for free. The other two you have to get with training points. So far, people think that the current Shadow Magic is an infected Moon Magic. Shadow Magic can change your form, just like Moon Magic, but it does not change your health or give you a new spell deck. Instead, it gives you awesome boosts. But, in exchange, you get Backlash. Backlash is an Angry Orb that hovers above your head. If you use the boosts Shadow Magic gives you, it lowers your Backlash by 10%. If you don't do what the Shadow Magic boosts give you, the Angry Orb gets bigger and you get an extra 10% Backlash. Backlash takes a percent of your current health and damages you. It cannot be effected by charms, shields, traps, or weaknesses. Currently, the Shadow spells are:
If you need more information, you can go to the Update Notes for November 2013 and read the Shadow Magic section (sorry I'm kind of in a hurry so I can put the link).