The most obvious goal of this deck is to make an army, but this army isn't to bolster our commander. On the contrary, the commander makes the deck look unassuming. They present a useful tool for resurrecting a specific creature, but overall they won't be a game-winning piece. In the end, our opponents are more likely to prioritize other opponents over us. In that down time, we can build our zombie horde.
We have several ways to make an army and bolster it.
Cemetery Reaper can allow us to exile a problematic creature from an opponent's graveyard to make a 2/2 Zombie, or it can be used on our own graveyard if we're desperate.
Diregraf Colossus makes a 2/2 Zombie any time a Zombie spell is cast. As mentioned earlier, this goes infinite with our "Crawler to the Altar" method.
Havengul Lich allows us to resurrect a creature from our graveyard. It also becomes that creature. This is exceptionally useful for getting useful, large creatures onto the battlefield.
Headless Rider can also be used with our "Crawler to the Altar" strategy to make an infinite Zombie Horde, but it can also be useful in the case of boardwipes. Wipe our nontoken horde? Replace it with zombies.
Noosegraf Mob works really well in this deck, as the buffs given to it from our Zombie lords (such as Zombie Master) and other possible buffs just gives Noosegraf Mob more potential to create 2/2 Zombies.
Sheoldred, Whispering One allows us to return a creature card from our graveyard to the battlefield untapped, meaning it can be used again. This is essentially Havengul Lich without the mana cost. It is extremely useful for bringing back powerful creatures and, if used correctly, can be used to essentially cast a mana-hungry creature for free. This works well with cards that keep track of things leaving the graveyard, like Tormod, the Desecrator.
While not infinite, we can pay and tap Lord of the Undead to return a zombie card to our hand.
Finally, Wilhelt, the Rotcleaver gives us the ability to create a 2/2 Zombie with Decayed whenever a Zombie we control dies. Again, this can go infinite with "Crawler to the Altar", but it works really well with other creatures. Further, it can be used to supplement lost Zombies or to simply add to the horde's numbers.
Of course, We have more than creatures to build our horde.
If the "Crawler to the Altar" strategy is combined with Necroduality, as mentioned above, it creates infinite Gravecrawler tokens. This addition to our horde's numbers is especially important.
While it doesn't directly add to the horde, Arcane Adaptation turns all non-Zombie creatures we own into Zombies. This means anything that relies on Zombies can be used on every creature we own. Non-Zombies, like Sheoldred, Whispering one; Erebos, God of the Dead; and even our own Commander reap the benefits of our zombie legion.
Several spells allow us to grow the horde. Army of the Damned gives us thirteen 2/2 Zombies. For two extra mana, we can create another thirteen 2/2 Zombies thanks to Army of the Damned's flashback cost.
The number of Zombies we control is extremely important. There are many ways to expand this number. One such way is Rise of the Dark Realms. This card is why we have Traumatize in the sideboard and Brain Freeze in the mainboard. Traumatize makes a target opponent drop the top half of their deck into their graveyard. Statistically, at least one creature should be in this cluster of cards. We then resurrect them under our control using Rise of the Dark Realms. When used alongside Crawler to the Altar, we can infinitely mill our opponents with Brain Freeze, then use Rise of the Dark Realms to turn their creatures against them, as once we run out of cards to mill on one opponent, Storm allows us to redirect the effect to the next, and then the next.
We have a planeswalker and an enchantment that present the greatest benefit to our horde: Liliana, the Last Hope and Endless Ranks of the Dead. Liliana, the Last Hope's ultimate ability gives us an emblem that lets us create X 2/2 Zombie tokens at the beginning of our end step, where X is two plus the number of Zombies we control. This is extremely valuable with many combos and spells we have.
This also applies to Endless Ranks of the Dead, which makes X 2/2 Zombie tokens at the beginning of our upkeep equal to half the number of Zombies we control rounded down. The entire "Crawler to the Altar" strategy can make these game-winning conditions (assuming the opponent doesn't have a boardwipe, or we have a counterspell).
Look at any infinite Zombie token combo above. Say you make 300 Zombie Tokens using "Crawler to the Altar", then you use Liliana, the Last Hope's ultimate, your number doubles to 602 Zombies at the end step. By the next upkeep, that number jumps to 903 Zombies thanks to Endless Ranks of the Dead. More reasonably, say we have 12 Zombies in the graveyard. We use Zombie Apocalypse to resurrect those 12 Zombies, then use Liliana, the Last Hope's ultimate. By the beginning of the end step, we end up with 26 Zombies. If Endless Ranks of the Dead is also on the board, we then gain 39 Zombies at the beginning of our upkeep. If we have Arcane Adaptation on the board, that number has the potential to increase substantially.