Here is the full list of all 19 DarkenDerek The last Fallen achievements.
You Have Purchased Darken Derek
Complete Level 1
Complete All Map Objectives 1
Complete All Map Objectives 2
Complete All Map Objectives 3
Complete All Map Objectives 4
Complete Level 5
Complete All Map Objectives 5
Complete All Map Objectives 6
Complete All Map Objectives 7
Complete All Map Objectives 8
Complete All Map Objectives 9
Complete Level 10
Complete All Map Objectives 10
Complete All Map Objectives 11
Complete All Map Objectives 12
Complete All Map Objectives 13
Complete All Map Objectives 14
Complete All Levels