I'm pretty new, but some advice I've picked up for Nighthaunt so far:
They're primarily a horde army, so you are going to want some big units. 2 lots of 20 Chainrasp is more or less mandatory. Consider a Chainguard Battalion if you go that route too.
Grimghasts look really good too.
You probably want 2 Guardian of Souls and probably a Spirit Torment too. Guardian of Souls seem to be what really hold the army together.
Black Coaches look so-so.
Probably also worth looking at Legions of Nagash, as they can take most Nighthaunt units and ally the rest - they have some very cool Allegiance abilities too. Not that it's better as such, just different and worth considering.