The Plague Cart was part of the Undead Army, but was also included in the Nurgle Army in the The Lost and The Damned (pg 206.)
The legends of all lands and races tell of the Plague Cart, but whether it is of this world or another, none can say.
It may appear on my battlefield at my time. Roll 2D6 at the start of each turn: on a 2 or 12, the cart appears In the middle of the left or right table edge (equal chance of either). It moves in a straight line across the table at 4" per turn, leaving at the middle of the opposite table edge.
The insubstantial cart may pass through obstructions and even units of troops. It causes fear in all living creatures within 6", and terror in my living creatures through which it passes. It is immune to non-magical attacks As it goes, the spirits rise from the slain, following it wailing and moaning.
The Plague Cart may be summoned by a level 3 Necromantic spell Summon Plague Cart (cost 12 CPs), passing across the battlefield as described above. It gains the following powers upon encountering an Undead army:
Undead creatures within 12" are immune to instability. Any living humanoid slain wIthin 12" rises behind the cart as an unarmoured Zombie with hand weapon, with the appropriate profile and special rules. Each model is marked (e.g. with plasticene) to show that it is Undead, and the resulting unit is controlled by the Undead player.
Special Rules: The carter is armed with a scythe and is a normal spectre.