No More Room in Hell....
They are tireless. They feel no pain, no fear, no remorse. In endless ranks they throw themselves against the defenses of the living, battering them down in a relentless tide of aggression. Their very presence blights the lands of the living. Even when they are destroyed, their tainted essence poisons the land around them. Few can escape their ceaseless hunger, as they claim victim after victim.
Always feeding, yet eternally hungry.
The First Victims
The infestation begins small. A broken crypt, a shattered grave. Cryptbreaker is perhaps the most important Zombie in the deck, certainly at the one-drop spot. With him, you can pitch cards like Gravecrawler (the other important one-drop!) or Nameless Inversion, which you can then bring back later. He can also tap to help draw you cards. You can do this on the opponent's end step, or when you block with a creature that will die anyway, or on a turn when Song of Freyalise goes off, and all your creatures have vigilance, anyway.
Hordes on the Horizon
Powered by Song of Freyalise for extra mana, the Zombie game plan focuses on simply flooding the board with Zombies. Wayward Servant drains life for every Zombie that hits the field. Plague Belcher causes your opponents more pain even when your Zombies die (again). Diregraf Colossus can come out as a fatty or help you go wide (or both). Lord of the Undead really helps you keep your Zombies coming.
Nameless Inversion works very well with both the Colossus and Lord of the Undead. It counts as a Zombie, so casting it creates tokens, and it pumps up your Colossus when it comes out. You can use it to pump up one of your big guys, but usually it'll be used as a kill spell.
Overrun by the Undead
Army of the Damned creates so many Zombies that getting rid of them safely is difficult. With Plague Belcher out, sweeping the board is hazardous unless it's a mass exile or mass bounce. With one Wayward Servant out, Army of the Damned creates a huge swing. With more than one out, it wins the game outright.
True to Zombie form, these undead do not care if they are wiped out. You can do it yourself to get the last bit of damage in if you have a lot of Zombies. Throw down Bontu's Last Reckoning and you don't care if your opponents' creatures survive, because Plague Belcher makes all your Zombies into damage. You can overwhelm your opponent without need of combat damage with these synergies.
Or you can throw out a Bontu's Get Rekt on a turn that Song of Freyalise hits its third counter, making the board sweep one-sided, and then swinging in with a bunch of indestructible, juiced-up, vigilant Zombies.
Zombie Help Needed
So, I'm not sure what to throw in the sideboard, considering I have Abzan colors to work with. Path, probably, and some enchantment/artifact hate. Maybe the usual stuff: Stony Silence, Kataki, War's Wage, some hand control, etc. But I'm curious about the main list, as well. I was considering putting in Josu Vess, Lich Knight instead of or in addition to Army of the Damned. The latter's a little cheaper, but the former can be cast via Cavern of Souls and is thus uncounterable. He's also a threat on his own, and so are the Zombies he creates. With Wayward Servant or Plague Belcher, Army is a massive threat, however, where you can get them coming or going.
That's the general idea of this deck: hack away at your opponents whether you are putting Zombies out or they're leaving the battlefield.