X-Blades

Purchase Dark Ball (130 Souls), Dark Rage (1,300 Souls) and Dark Form (13,000 Souls) to unlock the achievement.

Purchasing dark spells will make you unable to achieve the good ending achievements, so you'll need to do this on a separate save or play the game again to unlock those.

Once you finish the first level or so you should unlock lots of ablilites for you to buy with the souls you collect. Once you can do so, there is a spell called "Darkball". Buy it. Once you have bought it, a new spell will come up called "Dark Rage". Once again, buy it. Once you have both spells the 3rd and final dark spell will come up called "Dark Form". Once you buy this the achievements will unlock. After you buy darkform go and use it to unlock the achievement for using dark form for the first time - worth 20G.