Looks good so far. A few thoughts come to mind:

1. Vampire Nighthawk because he is almost one of the best cards ever printed. He's always a threat no matter when he comes out.

2. Drop the temples. You don't really need them, also they are so slow and modern is not a place for slow cards. I would suggest a Nykthos, Shrine to Nyx in there for mana ramp.

3. Either Grave Pact or Tribute to Hunger for destroy and life gain.

4. Nightveil Specter for stealing win cons from other people.

5. Sorin Markov because he decimates enemies.

Very good deck all suggestions aside, let me know what your thoughts are on those suggestion.

A lot of the cards you have mentioned in fact all of them, are ones I really like. I have a play mat of Nighthawk even. The idea was to maximize on the Gary devotion. Geralf's Messenger I felt fit the bill as the better three drop, instead of nighthawk or Specter. Mainly because of the immediate effect when it enters and then it's undying effect. Plus 3 black for my devotion. You talk of slow cards, and I think Nighthawk and Specter are slow for this deck. Messenger is just a little faster in the sense that it immediately effects the game. I will keep Nighthawk in mind though. Maybe it would be better.

On to Grave Pact and Tribute, again Tribute seems a lot slower than I want for this deck. 3 mana for opponent sac when I can make them sac for 2? Seems better to go with the two. And they lose a life. I have a 3 mana removal and that is downfall. Really solid spot removal. Grave Pact never crossed my mind though. I'm not even sure where I'd put it. Perhaps remove some of the swamps? idk.

Sorin also seems too slow even though he adds nice devotion. And Nykthos, I would put maybe 2 max. Good idea with that one. Will have to add.

I disagree with LtMiller117 on Grave Pact/Tribute to Hunger. I like your current removal, and you don't hvae a way to exploit Grave Pact in this deck. However, I totally agree with him on Nykthos, Shrine to Nyx . Drop all 4x Temples, and put in just 2x Nykthos though, because they're Legendary Lands and you can only have one on the field at a time. You really don't need 26 lands. I'd mainboard 2x Phyrexian Obliterators for those 2x left over slots when you can get them. Otherwise this looks pretty good buddy.

Ah. I see. If that's the case, then ignore a good portion of those suggestion(totally missed gets verdict). It looks like it runs really well as is. I would really consider nighthawk and nykthos heavily though, not to replace messenger but for solid defense and flight game. And I would put away the temples. Modern is a lot more fast paced than any other format and things coming in tapped aren't helpful. Hope some of these ideas help. :)

Also, I meant to put Grave Peril not grave pact. I is dumb.

Grave Peril makes infinitely more sense, but in the sideboard to deal with Tron decks.

Haha, yes. Just realized that mistake.

Well how about this. The 2 Nykthos in the main and instead of 2 Obliterators, 2 Vampire Nighthawk. As I had stated in description, the plan is eventually to replace Abyssal Persecutor when I get the money for Obliterator.

And thanks guys

Definitely better.

Dark Prophecy

Hmm. I'm not sure I run enough creatures to get anything out of that card. I like it, but It works better in a deck with like, Bloodghast doesn't it?

Bojuka Bog as a one-of or two-of against anything that relies on the yard, like Snapcaster Mage, Vengevine, Life from the Loam, ...?

Also, maybe Stillmoon Cavalier as a mana sink for all your black mana you will be getting from Nykthos, Shrine to Nyx? Also Erebos, God of the Dead as a one-of or two-off is a nice card for devotion and can function as both a finisher (if you have the five devotion for him) and card advantage in the late game.

lliliana of the viel.

Erebos is actually going to be a 2 of in my side. I've got a single Veil and I think what I might do is one of every Liliana when Heretical Healer comes out. I'll consider Stillmoon in side too. Bog should be fine in the board. Gonna look through

I highly recommend Blood Artist to make every time any creature goes to the graveyard a life drain. On top of that a Bloodthrone Vampire or preferably the zombie version Nantuko Husk will allow you to keep sacrificing and pumping these creatures. like if you had enough devotion you could summon a Gravecrawler to the battlefield and sacrifice it 6 times, (or as much devotion as you have). you could sacrifice a Bloodghast before dropping a land to have a +2/+2 on one and a haste on the other.

Instead of Phyrexian Arena maybe Underworld Connections because then you can choose weather or not to use the ability. as long as we're looking at enchantments I suggest the cards Palace Siege and Shadows of the Past. Any of these are ok replacements for Read the Bones, (but it is still probably the best.)

Bone Splinters works well to both trigger Geralf's Messenger and flip lilliana.

For example: T3 play lilliana, T4 summon Geralf's Messenger to deal 2 damage to them, then Bone Splinters to sacrifice geralf's and one of their creatures, Next geralf's returns to the field dealing another 2 damage to them. You could go a step further in T5 with a Cruel Revival to return another zombie to the field. I should mention Tragic Slip is also amazing removal in this type of deck.

However if you want to stick to just devotion: Killing Glare, Death Wind, Exsanguinate, Death Denied, Mogis's Marauder, and Abhorrent Overlord.

Good luck!

So instead of working today I've spent the afternoon playing with your deck, i really liked your idea! Here is a Copy with altered cards. Modern black devotion copy Linkdude74

I also left all my maybe picks in there too. The biggest problem I had was that the low 1 and 2 drops couldn't block, and Geralf's Messenger came in tapped so it was T4 before you could block so you would die before you could get out the win condition. So i put in one and 2 drop creatures with deathtouch to discourage attacking, then I threw in searching cards so i could grab any one of the win conditions or the Nykthos, Shrine to Nyx. I also went with a Liliana of the Dark Realms as her Mana production seems better for this deck.

Take a look and I hope it it helps.