Hrm. The
Undead formation consists of a mortis engine, a single unit of spirit hosts, and a single unit of hexwraiths. In each hero phase, you can choose to remove either the hosts or the wraiths from the table, and then re-deploy them w/in 6" of the engine but at least 3" from enemy models.
It's... well, it's an interesting formation. I'm sure there are clever tactical aspects to it - perhaps deploying the spirits on one side of the table do draw your opponents attention to that flank, only to later deploy the engine on the other side of the table and pull the ghosts over in the first couple turns; or set up a wall of ghosts to halt an enemy melee units charge, then pull surviving hosts out of the way to clear room for a counter charge by something else; or pull hex wraiths out of combat so they can use their fly by attack in the movement phase before charging back in; etc. Would be stronger if they had some charging bonus to benefit from recharging every turn like the lizards formation does (it allows the units to 'reform' around one of its models, potentially pulling the unit out of combat, then move normally, potentially charging again). Still, redeployment rules like this tend to have interesting applications that aren't immediately apparent. That said...
It's not the big upgrade for the mortis engine to give it that 'wow factor' I currently think is a bit lacking in its rules, at least compared to the model's physical presence on the tabletop. And I have no idea how it compares to the other formations (apart from the lizardmen one, which looks like it would tear these ghosts apart), which, again, are all probably a fair bit stronger than this one if played straight out of the box, in particular considering that this box has no real heroes or spell casters. But with slightly larger unit sizes, and in a force where this formation wasn't the only thing your were running, I could see it being pretty useful.
Again, I won't be picking up the box set itself any time soon, as I already have a mortis engine, and am not in any particular rush to add a coven throne to my collection, but I might proxy the formation with my black knight to try it out, and maybe grab a unit of hex wraiths if I like out it plays.