It's standard to mention that weapons you use as implements do not give Proficiency bonus to implement powers. It's part of the rules, but they do repeat with every single ability.

Demonic Presence is probably too strong. Bard has to invest quite a few feats to get something close to that effect.

Right now I see absolutely no reason not to take the biggest weapon you can get, for large damage boosts. Also, doesn't make sense with bows. Perhaps specify it must be a one-handed melee weapon?

Vile Blast is a bit too good, drop Save ends.

Demonic Heritage is fine

Unending Depths is fine.

Overall looks good otherwise.