The Maghda's Tormented Soul is a random drop for killing Maghda. I looted about 5 of them in about 40 kills. Simply create a normal-difficulty Adventure Mode. Take the waypoint towards Act II: Road to Alcarnus. Enter Maghda's lair. If she doesn't drop it, leave the game, and rinse and repeat until she does.
Cain's Destiny Set
The Maghda's Tormented Soul is required to craft the Cain's Destiny set at Blacksmith Haedrig Eamon. The Cain's Destiny set adds +6 magical properties, and as you can see from the images below the stats are really good with +100% Magical Find, +50% Bonus Experience and +8% attack speed. On my Crusader level 70 (Paragon 32), I currently get a lot of 1.5 million crits at fast speeds using runes that trigger attack speed.
Note: The Ascended armor required to craft these items can be farmed at the Act V: Battlefields of Eternity in Pandemonium. They drop off the armor racks, and loose stones in normal difficulty.
[table id=70 /][/bluepost]
[table id=69 /][/bluepost]
[table id=68 /][/bluepost]