Here is the full list of all 9 Knight Terrors achievements.
Reached level 10 in Normal mode
Unlocked all powerup weapons
Unlocked all modes
Stacked all weapons at once
Invoked the Spirit of Nashwan
Obtained the maximum number of heart containers
Reached level 5 in Knightmare
Scored over 300,000 in Normal mode
Scored over 1,000,000 in any mode