I like to come up with narratives for my characters and armies, which helps inform my unit and character selections. For example my Vampire Lord, Wolfram von Blutsauger, is a very proud and martial lord who embodies the aristocracy of the night and draws a good deal of inspiration from good ole Dracula. So he has lots of creatures of the night (bats and wolves) in his force, but views the more animalistic undead (Ghouls and Crypt Horrors) as beneath him. He lacks the magical finesse (being only a level 1 wizard) to adequately bind the more magical undead (banshees, wraiths, hexies, spirit hosts) to his will. He has a cabal of indentured Necromancers to provide magical support to his forces, and surrounds himself with sycophantic lesser Vampires, whom he mercilessly manipulates and abuses. So there is also a rotation of lesser Vampires in the army, as well as a unit of Blood Knights. The rest of the army is rounded out by zombies, skeletons, black knights, and von Blutsauger's pride and joy, Maleficus the Zombie Dragon.
I have a narrative working it's way through my mind in regard to von Blutsauger's goals and motivations. Something about a lost love, taken from him and destroyed, such that he seeks vengeance and a way to restore her to unlife. In larger battles I'm thinking of including a Black Coach with the remains of his love inside. Or that she was taken and entombed ala Vargheists, and when he went to rescue her she was gone but a unit of Vargheists attached itself to his forces. This lets me rationalise battles against almost everyone, as he seeks relics of great power to enact the ritual to restore her to unlife, as well as the more mundane day-to-day battles associated with running a secretive Vampiric kingdom. Keeping the cattle in check, sowing fear and confusion, fighting off rival Vampires and other undesirables, etc etc.
I also like to think that one of the lesser Vampires attached to his court has an infatuation with him, and she seeks constantly to undermine his efforts to recover his lost love. He is, of course, aware of this but tolerates her interference as long as it remains low level. His relationship with her is complicated, as he feels a certain affection for her, but only in so much as she replaces his true love. She adores him, but at the same time would rather see him destroyed than reunited with his true love.