It makes no sense to me why Major Prophecy is on Bound Armaments, which is a Daedric Summoning skill. This causes 2 main issues:
1) Dark Magic providing Minor Prophecy, yet lacking a source of Major Prophecy. Compared to Assassination which provides both Major and Minor Savagery, we can clearly see a disadvantage when choosing Dark Magic skill line
2) Limited options for single subclass builds. You either have to give up Minor Prophecy or be limited to running Grave Lord/Assassination if you want a damage subclass line.
I think moving Major Prophecy to Crystal Shard and its morph would clean up this issue.