The character is really difficult to fit into lore due to there being so little to base off of. We hardly know anything about the shadows and the hierarchy of them. That saying I will try my best to explain and as always its subject to change as new lore is revealed.
Velthus is still a very unknown being. It refers to itself in plural terms ( us,them,we) and mostly talks in multiple inaudible whispers. Velthus holds a vendetta against the beings of shadow from past conflict. (which is why the Codex Lux is damaged)
For now they are kind of on the back burner and mostly is there just to be the one Wraith hears and is tormented by. Velthus feeds terrible visions and knowledge to Wraith in hopes of breaking her willpower.
The Codex Lux allows the reader to draw upon Velthus' power and manifest fragments of light.
Hopefully that kind of, sort of makes sense,