In the Awakening you get to choose the difficulty level in the beginning of the game. It can be changed later at any point in-game in the inventory.

Easy
Enemies' Stamina regenerates slower, the player's Stamina regenerates faster. All attacks you do deal 10% more damage.

Medium
Enemies attack with more damage. Everything else remains at their default values.

Hard
Enemies' Stamina regenerates faster, the player's Stamina regenerates slower. All attacks you do deal 10% less damage. Enemies attack with even more damage.
If you complete the game on Hard, you get the Soulcleaver sword at the end.

Super Easy
Can only be selected by cheating. Enemies' Stamina regenerates much slower, the player's Stamina regenerates much faster. All attacks you do deal 20% more damage.

Super Hard (Available in V 3.5 or higher)
Can only be selected by cheating. Enemies' Stamina regenerates much faster, the player's Stamina regenerates much slower. All attacks you do deal 20% less damage. Enemies deal a lot more damage and have a lot more health.