Deck looks awesome! It reminds me of Liliana's deck in M14. I think 4x Nykthos, Shrine to Nyx is a bit too much. Your deck can get the devotion for it to be effective, but you shouldn't be running a playset because in some cases it won't generate enough mana. Endless Ranks of the Dead looks cool, but i think you could remove some creatures to put more Duress or control spells.

GidEmrakulPlox, the Nykthos have been helping me a lot, actually, as all the ords have BB on the cost, it escalates incredibly quickly. Also, from 3 CMC and up nearly all the cards have generic mana on the cost, so it rarely hurts

Ive tried to take something out for a playset of Duress, but the problem is that I would end up either having to cut a 2 CMC, and I dont have a lot of those, or cutting a 3 CMC creature, which are the main ones on the deck (lords, token generation and a couple finishers). What do you think should come out for a couple extra?

I kept it on the SB to add agains specific decks (usually with Extirpate alongside it) but most times I just want to keep attacking with bigger and bigger creatures. I would gladly take some out for AEther Vial, but those are a bit expensive right now, and this deck is actually pretty budget on the online world (the differnece from the actual cards is just ABSURD; $255 turning into $38)

Thruth be told, zombies is still not quite on par with merfolk or elves. merfolk lords give islandwalking (which they generate) and cost 2 mana, while elves basically ramp themselves. With zombies you are aways a little behind on the tempo...