Black mages are always looking for more power, at any cost. When looking through black cards to use in your deck you can probably find some with very powerful effects but these also come with various drawbacks. These can be things like losing life to draw more cards, seen commonly in cards like Sign in Blood
Sign in Blood, to Abyssal Persecutor
Abyssal Persecutor, gaining a 6/6 flying trampler with the drawback of not allowing you to win the game. M12 gives black mages a cheap 1/1 creature that is unblockable. However, this creature is so focused on attack your opponents that it can’t block at all.
Tormented Soul gives black mages a way to consistently get damage through to your opponent. If your opponent doesn’t take out the Soul then they are put on a clock, having at max 20 turns to win the game. That’s certainly plenty of time for your opponent to pull some answers, especially if all you have is a little 1/1 that can’t block. Of course, any clever planeswalker can make use of this guy to pull off some easy wins.
His appearance in M12 goes great with Bloodthirst, giving those creatures with the ability those extra +1/+1 counters they need without needing to waste a spell or worry about the creatures being blocked. In a black deck this can power up all of your vampires, especially with the Bloodlord of Vaasgoth
Bloodlord of Vaasgoth. The clock will continue to wind down for your opponent and that continuous damage will only make the clock go faster with the addition of more powerful bloodthirsty creatures.
Even on his own, the Tormented Soul can be pumped up to become a fearsome creature on his own. Equipment are great additions, able to be swapped to different creatures when needed. Even something as simple as the Greatsword
Greatsword can give a huge boost to your unblockable creature. A 1/1 is still pretty easy to beat with any spell that deals damage. Giving him shroud or hexproof with cards like the Swiftfoot Boots
Swiftfoot Boots and Vines of Vastwood
Vines of Vastwood can give your Tormented Soul the extra power needed to win the game for you. If you need lots of extra power to turn the game around Tormented Soul is also a great candidate for the Worldslayer. Sometimes you just need to destroy the world in order to earn that victory. That is certainly no problem for a planeswalker who dabbles in black magic.