Bone Splinters is pretty strong as well.

Yeah good call. I was debating whether to throw it in or not. I think I will.

Have you considered Altar's Reap?

What about Cover of Darkness, maybe you should at least keep it on the sideboard, as a substitute to Graf Harvest if your opponent is not running Black or artifact creatures. Also, why not have at least 2x Prized Amalgam? It would go very nicely with Gravecrawler and Relentless Dead. Maybe you could substitute Diregraf Ghoul for 2x Cryptbreaker and 2x Prized Amalgam. What do you think?

Endless Ranks of the Dead might be fun. Though it might be to slow.

I played Mikaeus in EDH for a year and a half. This reminds we so much of that deck. I love it!

Languish would be super strong with Mikaeus out. Sweep everything(ideally) and then have all your creatures bounce back with a +1 +1 counter. Also maybe sub out Murder with Hero's Downfall for Planeswalker removal as well

Unfortunately this deck is Modern legal so Cover of Darkness isn't an option, and due to it's tempo and restrictive mana costs splashing Blue is just going to slow it down. I did, however, toss in Cryptbreaker, as it meshes well with this deck. Endless Ranks of the Dead is flavourfully one of my favourite cards... But it's too slow here, though it has a place in my Zombie-themed control decks. I used to run Mutilate as opposed to Languish but deathtouch and spot removal do a good enough job, though I leave it in my sideboard. Mikaeus just doesn't see enough action to justify putting in any effort to abuse Undying. Good call on Hero's Downfall, totally forgot about it.

What about ghoulcaller's chant

GCC is an excellent card, but I've found that the pace of this deck is supplemented more effectively by a good balance of card draw and graveyard antics. So I went for both with Cemetery Recruitment.

I'll spare a lengthy explanation but Diregraf Colossus is truly a monster and offers so much synergy.

Would Undead Warchief fit in the deck?

I'd cut Graf Harvest and Cemetery Recruitment to 2 or 3 each. Both cards can potentially be dead draws and aren't 100% necessary to your strategy. This type of aggressive tribal could be a good shell for AEther Vial to keep up Mana for Relentless Dead, Cryptbreaker and Lord of the Undead on your opponent's turn as well as any Instant cards. Other than that, I think Go for the Throat is better than Ultimate Price. Only if the people you usually play with run lots of Affinity decks I'd prefer the Price. Victim of Night might even be better than both, your deck being Mono-Black and all. Cavern of Souls, Nykthos, Shrine to Nyx and Mutavault are also great lands you could splash in here. Geralf's Messenger if you take the Aether Vial route and can afford a clunky 3-Mana heavy curve that way. Also think about some Maindeck hand destruction, a well timed Inquisition of Kozilek, Thoughtseize or Collective Brutality can often ensure the win over combo-heavy decks, boggles, infect etc.

Also, ignore my comment about Ultimate Price and Artifacts. It doesn't hit colorless

Undead Warchief is in my Legacy Zombies deck, it's too slow for this deck if it wasn't Modern legal anyways.Very well thought out ideas casmiel. I will definitely find a spot for Collective Brutality, the flexibility of that card is fantastic, plus the discard outlet will work to offset the extra copies of Graf Harvest and make sure I'm always drawing a card with Cemetery Recruitment. I used to run Go for the Throat and Victim of Night but my meta is heavy on Vampires and Affinity so I've found Bone Splinters and Tragic Slip do the trick rather well in this deck.

I think you should cut mikaues in play testing when you can cast him you have already won.

And undying does nothing about path to exile.

Cavern of Souls and AEther Vial?

Plus Cemetery Recruitment and Gravecrawler do not work together.

I'd run Cavern of Souls if it wasn't like 60 bucks per card. I've actually been running Sign in Blood instead of Cemetery Recruitment, forgot to change my board.

Other cards to consider:

Liliana's Reaver, Zombie Master, Sidisi, Undead Vizier, Zombie Trailblazer, Grave Titan, Army of the Damned, Risen Executioner, Geralf's Messenger

Obviously some/most of these cards will be horrible in your deck, but others you may want. Pick and choose.