Keep in mind that decks that perform well in standard do not necessarily perform well in older formats. That said, if this were standard, it would wreck many more faces that the actual standard variant already does.
is it cause im monoblack??cause if so thats RRRACIST!! or if not how could i make it a suitable mono black in modern?
im thinking maybe Liliana of the Veil ....
maybe bloodghast? Gives 2 devotion and recurs like a mofo haha. Also...why not thoughtseize? For fun, you could also use sheoldred :P
And I honestly prefer Underworld Connections to Phyrexian Arena
I love Sanguine Bond in devotion decks!
The thing about modern is mana cost.. typically who can play the fastest...you want to keep mana cost around 2-3 average for most of your cards. Yes its okay to have those high drops because typically mono black is mid-range, but you have to make it there. Discard is really important for black, so consider adding maybe something like Mind Slash or Wrench Mind .
You have way too many cards that fights for your 4th and 5th turn plays. Also, Underworld Dreams is just ineffective in such a format, people play fetches and shocklands, along with Dark Confidant and Phyrexian Arena . Making them lose 1 more life on the third turn at the earliest isn't really an effective strategy. You should probably lower your CMC and focus on Gray Merchant of Asphodel and Demigod of Revenge , along with hand distruption like Thoughtseize , Inquisition of Kozilek , and maybe even 2 Duress . Path to Exile do not play well with your demigod.