It’s an interesting relationship because Neph probably won’t abuse her position over sunny, but also she shouldn’t release him from his enslavement because it would leave him vulnerable to enemies like Mordret who know his true name.
seeing someone in there dreams, (literally) is one way of having a private rendezvous.
if he could take others on nightmare back, into dreams This would be a very useful methods of communication, or torture with even is more convenient.