The Amazon 10 Days of Deals is in it’s fourth day and today’s deal is on Magicka. The game is on sale for $4.97 ($9.99), which is half off. This game activates on Steam. In fact it requires Steam. This matches the price that Green Man Gaming had it at over the weekend. (Actually it’s $.02 more).
When you purchase, you’ll get a cd key that you activate on Steam. You’ll then download the game through Steam and then you can start playing. Very simple.