you should add 2 copies of Urborg, tomb of Yawgmoth (for the cabal coffers, it makes them far more potent) also why not have a look at unholy grotto, korlash, heir of blackblade loves himself and his pals some unholy grotto's and lord of the undeads.. If you do choose to add the Urborgs, Zombie master is an big Urborg lover, and so am I I've had quite some experience building zombie decks, so there's some tricks I know to get them to be quite deadly.
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Why did you maindeck the grafdigger's cage? It doesn't work all that well with Sheoldred. What happens if you don't draw the heartless summoning? Because it's virtually the only card setting you up for bringing the big guys in (excluding Simulacrum, but he's also 4cc).