Archfiend Zombie-Skull is a Level 6 Synchro that prevents face-up Zombie monsters you control, including itself, from being destroyed by a card effect, protecting them from cards like Desperado Barrel Dragon and Treacherous Trap Hole. Unfortunately this Synchro Monster a specific Tuner (Plaguespreader Zombie) and two non-Tuner Zombie monsters in order to be Summoned.
I would advise using this method only if you have a Zombie monster in your Graveyard to banish for Gozuki’s effect. Vampire Familiar, upon being Special Summoned, will allow you to search a “Vampire” monster that you will be able to Special Summon when Gozuki is sent to the Graveyard for the Synchro Summon. Definitely a convoluted way of Summoning a single Synchro Monster, but it’s not like this Deck cannot function when this play is not available.
Vampire Kingdom will increase the ATK of Archfiend Zombie-Skull by 500 during damage calculation, making it impossible for Desperado Barrel Dragon to destroy it by battle.
Doomkaiser Dragon is another Level 6 Synchro that requires Plaguespreader Zombie in order to be Summoned, but, unlike Archfiend Zombie-Skull, it doesn’t force you to use at least two non-Tuner monsters, which means that you can Summon it using only two cards: Gozuki (or Samurai Skull) and any card that can be placed from your hand on top of your Deck.
When it’s Special Summoned (not just Synchro Summoned), Doomkaiser Dragon allows you to target a Zombie monster in your opponent’s Graveyard and Special Summon it to your side of the field in face-up Attack Position, but destroy it when Doomkaiser leaves the field.
Well, the most relevant Zombie monster that you will find in your opponent’s Graveyard naturally is going to be Bacon Saver, but, although getting rid of your opponent’s ability to negate any attack can still be considered decent, we are not running this monster for this reason. Thanks to Zombie World and your Skill, Posthumous Army, you can Special Summon any monster you want from your opponent’s Graveyard.
Running both Gozuki and Samurai Skull may seem redundant, but it’s necessary: it’s the easiest and most consistent way of getting Vampire Retainer and Vampire Familiar into the Graveyard.
You can use Retainer’s effect to search the Vampire Kingdom, Vampire’s Desire, Vampire’s Domain or Vampire Takeover. If you want to Summon Vampire Vamp or Vampire Grace, you are likely going to need the additional Normal Summon/Set from Vampire’s Domain, whereas, if you already have the Continuous Spell available, you might want to go for your Field Spell.
You can use Familiar’s effect to search Vampire Grace, which, in combination with Vampire Kingdom, used to be this Deck’s primary way of removing potential threats from the field, or to add to your hand Vampire Vamp, a Level 7 “Vampire” monster that can get rid of high ATK monsters like Elemental HERO Brave Neos and Blue-Eyes White Dragon by equipping them to itself.
Unfortunately, if they were Special Summoned by their own effect, both Retainer and Familiar are banished when they leave the field, which means you will not be able to Special Summon them again on your next turn or even to use them for Gozuki’s effect. You can use Burial from a Different Dimension to replenish your resources, but I haven’t found particular success with that card.
Aside from having obvious synergy with Doomkaiser Dragon, Posthumous Army has also the ability of preemptively disrupting your opponent’s plays. Turning all of your opponent’s monsters into Zombie-Type monsters means that they are not going to be able to Special Summon Desperado Barrel Dragon, which requires a DARK Machine-Type monster to be destroyed, Legendary Six Samurai - Shi En, which requires a Warrior-Type Tuner, or even Silent Magician, which requires a Spellcaster-Type monster. All you need to activate this Skill is lose 1000 Life Points, which is something pretty easy to do in a Vampire Deck.