What do I need to level this up a bit. Pack Rat ? Some control spells?

For mono black devotion, you will want: Underworld Connections Pack Rat Thoughtseize (probably too expensive: budget version is Duress ), Hero's Downfall , Nightveil Specter . Those are all suggestions that would make the deck more competitive and much more costly,

29 creatures isn't really a good thing, and neither are 4 sorceries...anyone can clear the field with Supreme Verdict And they can remove Erebos with Turn / Burn or Pacifism See which creatures you can cut, especially those that don't add to devotion.

Desecration Demon is a great addition. Also, use Corrupt as a game-ender. Liturgy of Blood is one great removal spell, as well as Hero's Downfall . Mind Rot , Duress , Thoughtseize are all pretty good as well. Get Nykthos, Shrine to Nyx . Sideboard some return-creature-from-yard (a.k.a. recursion) spells.

For mono black do you think it's better to get more creatures out quickly to build devotion, or keep the spell count high to keep your opponent controlled?

It's better to get creatures out in the beginning. However, use more 2 drop and 3 drop creatures, such as Vampire Nighthawk , as most cost more than three mana. With your deck, in this state, have more control spells.

Also, thinking of adding Rise of the Dark Realms to your sideboard?

although it may be out of your budget might i suggest a Liliana of the Dark Realms to accompany your Corrupt and maybe even Crypt Ghast for life and extra mana.

I like the Liliana idea, it's always nice to get guaranteed land drops every turn.

I have a black and blue deck like this mine still needs a lot of tweaking. But I love the Sanguine Bond because it doubles your opponents damage from Gray Merchant of Asphodel and if you are playing 4 player it causes 3x the damage to all players. Geralf's Messenger has high devotion to black. And I have 4x Rescue from the Underworld which allows you to be a more aggressive attacker for you can rescue cards from your grave yard. I love playing it when I have a Asphodel in the graveyard and an Asphodel in play. Bring them out of play and then back and double the special ability. A great combo is also Abhorrent Overlord then during your next upkeep sac it and then bring it back with Rescue from the Underworld over and over, you'll have a flying nightmare. This is my first post, and I'm just getting back into the game after 15 years, I hope it helps.

Caustic Tar is a 6 drop that does almost nothing, get rid of it