I would highly recommend against running Aether Vial in the sideboard. You just don't have the right creatures for it. For Aether Vial to be strong, you'd need a much much tighter mana curve on your creatures (90% 1-3 CMC, maybe a couple of 4 CMC creatures MAX). I'd rather run a couple of Collective Brutality to vastly improve your chances against Burn and some more Leyline of the Void.

As for your maindeck, I'd swap Underworld Dreams out for Phyrexian Arena. Dreams is only strong in a very narrow range of matchups and you have Howling Mine effects or similar to take advantage of it. I'm also not sold on Nether Traitor, apart from the Devotion he can't block for you and 1 chip damage is just pretty much nothing without pump spells. Rather run the full set of Gifted Aetherborn and a couple of Victim of Night over them. Victim is pretty strong in Modern as there are few commonly played creatures that resist it, most of them are found in Dredge where removal is pretty terrible anyways. Other than that, I'd swap the Despise for Inquisition of Kozilek. Inquisition is more flexible in most matchups and you have tons of ways to deal with creatures anyways. With Artifacts, Enchantments, Instants and Sorceries? Not so much.

I also think your Liliana is versatile enough to run in the Maindeck.

Love me some monoblack, nice job

I would play Smallpox because it disrupts everything and you can recover quickly with your returning creatures. Torment of Hailfire as a 4 of seems a bit much as it won't do anything for 2 mana, next to nothing for 3 and 4...it requires too much mana to have it show up frequently in the opening hand imo.

Winterblast, With Nykthos, Shrine to Nyx on the board, Torment of Hailfire can be cast easily to do massive damage. In play testing, I have routinely had access to 9 mana on turn 5 when I have That land in the board. The devotion in this deck works very well. For right now, since it's a new card, Torment of Hailfire won't likely be subbed out. Smallpox is definitely an option though.

OK, so, there are more efficient ways to build the creature devotion.

Bloodghast is easier to recurr than traitor, and you can play fetches easily. Gatekeeper of Malakir is actually pretty solid given a lot of the DS decks want to play 1 creature and protect it, and this kills that creature and blocks. Relentless Dead recurrs itselfs, Merchant and Messenger which is hot. Geralf's Messenger is generically good devotion. Phyrexian Obliterator is a bit iffy, I don't like it all that much but it sees play as a 2 of. Gray Merchant of Asphodel should be a 4 of all the time to be quite honest. You want to see multiples and play on curve.

Liliana of the Veil & Liliana, the Last Hope are also awesome, but I would go for like a 1/2 split veil to last hope, as last hope does more in this sort of deck.

Fatal Push exists, add in the fetchlands for Bloodghast and everything in the format dies. Maybe some Dismember to, or if you add in some Tombs, Abrupt Decay.

Despise isn't run in modern because it just isn't that good, being conditional where as Thoughtseize is unconditional and IoK hits 90% of the targets in the format.

Grave Pact may give 3 devotion, but it doesn't do anything really. They just play around it. The decks that care about this can sacrifice their mana dorks, and the other decks have Path to Exile and similar.

Grotesque Mutation is just bad. Hero's Downfall is probably better as Never in your sideboard.

You saids Vesuva works well with Nykthos, how do you mean?

4x Fulminator Mage are must haves for the sideboard.

Most devotion lists will run 3-4 Phyrexian Obliterators. I also love a 1 or 2 of Sidisi, Undead Vizier. He has a big butt, profitably trades with just about everything, does not die to Fatal Push, and can easily tutor whatever you need.

In mono black is Urborg, Tomb of Yawgmoth really worth it without Cabal Coffers? Also, I would recommend Gatekeeper of Malakir he provides some versatile removal.

I think the lifegain plan can be dropped, Gifted Aetherborn is a great card and should stay but Divinity of Pride and Grotesque Mutation can be dropped to address a big need of yours: card draw. I would recommend budget Bob, Asylum Visitor.

The Urborg is to correct the Nykthos in the case I need true swamps.

Sup PickleNutz, it's ya boy here. I would say you're definitely gonna want to play four copies of Gatekeeper of Malakir, it can edict your opponent and gives you two black symbols for Gary. I would take out Gravecrawler for it since it's a very conditional card that isn't always that good. I would also probably cut the number of Nykthos you run since you don't need a ton of mana for stuff, but you do need almost all your mana to be black. Also I'm not sure if it's in your budget or not but having some noncreature permanents like Phyrexian Arena and Liliana of the Veil to provide you devotion that doesn't get swept away by Supreme Verdict. Also I saw someone else suggesting Bloodghast which I think is a solid suggestion. Fatal Push is also almost a necessity in any black deck I think. And since Gary is your primary pay off for devotion, I would bump him up to three copies. Also if you can afford Bob's, some number would be good. Brain Maggot can also be good in these types of decks. You could cut Grave Pact, Grotesque Mutilation, Despise, some number of Heroes Downfall, and some of the random creatures. Also I don't think Erebos is that great unless he's just a 1 of really.