One of the iconic units in any Undead army are the werewolves - fast and hard hitting they can fill the same role as heavy cavalry but are slightly quicker, more manoeuvrable, and not troubled by spear phalanx.
There were enough figures in the Undead mega army to do a Horde and a Regiment. This is boosted to 2 Hordes using some Owlbears as stand-ins.
I've also completed Lykanis, a melee hero who can Inspire the werewolves. A useful addition to a werewolf force.