Games Workshop has previewed changes coming to an army in Warhammer Age of Sigmar. The new Warhammer Age of Sigmar Nighthaunt Battletome brings the ghostly army into the game's newest edition, bringing some rules and mechanics updates along the way.
The Warhammer Age of Sigmar Nighthaunt Battletome Preview
In an official Warhammer-Community post, Games Workshop previews some of the changes coming in the new Warhammer Age of Sigmar Nighthaunt battletome.

Some of these changes are buffs and quality-of-life improvements. The units will retain their Ethereal rule, but now ignore just negative modifiers... except for Nagash. To balance for this, saves are now reduced to 5+ for Hero units and 6+ for spectres. But, to help reinforce the idea of the Nighthaunt as incorporeal warriors, they are all getting a 5+ Ward save and an additional unit of health per model.
The Warhammer Age of Sigmar Nighthaunt Battletome introduces new passives and resources. Mounting Dread debuffs the enemy's control scores based on how long the battle has gone. Etherreal Translocation lets you teleport a unit once per turn away from the enemy. Vanish and Reappear grants units bonus movement and lets them ignore terrain or enemy units.
The new battletome also includes new heroic traits and artefacts. Undead wizards can now use Wrench Souls, which can move friendly Nighthaunt units outside of movement phase, Alternatively, they can burden the enemy with Chains of Death, burdening them with strike-last for the rest of the turn. Then there's the unlimited spell ability, Fell Blades, which subtracts 1 from enemy save rolls against subsequent attacks.

Lastly, the Warhammer Age of Sigmar Nighthaunt Battletome preview features a new unit: the Lord Vitriolic. This ghastly potion-maker's Spectral Alchemy ability lets you provide a buff to your army. Spectral Solvent dissolves anything with a negative move characteristic. Choking Vapours debuffs enemy Wizards. And Unholy Prescription prevents the enemy from making Ward saves for a single turn.
All of these changes and more can be found in the new Warhammer Age of Sigmar Nighthaunt Battletome, which will be available for pre-order this upcoming Saturday.