When considering the most effective and efficient way to devastate a city in Creation, always be sure to ask yourself the following questions.
Is there any reason infecting the place with plague wouldn't work? And if plague won't work, are there any other fun diseases - like ebola from the South East - which I could give them instead? And once the place is plague-riddled, wouldn't it be fun if I also put cholera in the water supply?
Disease is your best friend as a hypothetical deathlord. Disease ruins mortal societies, causes the breakdown of civilisation, and hampers any attempt to rally a recovery against you because any army they'll form against your invaders is a nexus for disease. This is especially effective against Whitewall, which is overpopulated due to the limits of its city walls and which - should people flee the city to escape the disease - opens their refugees up to being slaughtered by the undead that already roam around the place.
Then once the place is sick and dying, get your Abyssals inside to invite your undead hordes in, and raise the dead within the walls. They can't mount a defence if everyone is ill, their walls are letting your undead through, and there are already undead inside the walls attacking them from within which the walls can't stop.
Abyssal and Deathlord tactics should be optimised around murder, not conquest. You don't care about their infrastructure. You don't care about their civilian population. You don't need people to work the land. And that puts you under rather different tactical constraints from living armies, which you should leverage mercilessly.
