I feel that
needs some mechanical changes that solidify his theme as a champion. His current skillset reflects his nightmarish theme well but doesn't mesh well in practice.
Umbra Blades (Passive) doesn't make sense thematically OR mechanically when compared to the rest of his kit. While I do believe he should retain some sort of sustain passive, this could use a rehaul to fit better.
Duskbringer (Q) is absolutely perfect. It instills a sense of dread because you KNOW what's going to happen when you've been hit and the trail is behind you. Only change I could ask for is to make the trail the projectile leaves branch off onto enemies that walk over it (no damage, though).
Shroud of Darkness (W) feels half-baked mechanically simply to push some sort of staying power for Nocturne but it doesn't truly interact with the target he's attempting to haunt which feels weird. Something that allows sticking power that still translates into a feeling of nightmarish helplessness is needed. Maybe an activated ability that turns him invisible for a very short duration. Duration increases if his Ult is active or he's currently standing on Duskbringer's trail. The first attack out of the invisibility causes a 1 second silence maybe?
Unspeakable Horror (E) is great. Current suggestions for lowering vision radius as the nightmare closes in would be fantastic though not altogether necessary.
Paranoia (R) is perfect. Nothing I'd change save for maybe increased movement speed on Nocturne while it's active.