Grim Flayer and Tarmogoyf are the main beaters in the deck. As such, i run four of each. By running the number of card types that I do, Grim Flayer can quickly and easily become a 4/4 trampler who's ability to fix your draws is nearly unparalleled. Enabling delirium with this deck is so easy and is even achievable as early as turn 2. Due to the synergy with my delirium subtheme Tarmogoyf also can quickly and easily grow to be a 4/5 and even has the potential to become a large as a 8/9. For two mana I cant complain.
Kalitas, Traitor of Ghet is run as a singleton to answer a number of issues. Primarily, he is there to serve as an alternative threat that does not rely on my graveyard. Additionally, he helps mitigate dredge decks and decks that like to abuse the persist mechanic and Vizier of Remedies (these GW combo decks are very prevalent in my meta) or other self sacrifice decks, he helps gain life, and against the right decks can by himself run away with the game.
Tireless Tracker I run two of them. Left unchecked, they can amount to a significant number of extra cards drawn (through the clue tokens) and grow to a significant size.
Courser of Kruphix is a creature I run two of. It is great for helping stabilize the field against aggressive decks and also provides incidental lifegain and card "draw." Also provides additional sources of "enchantment" when it dies in addition to the creature type.
Architects of Will serves as an artifact source that can easily be sent to the graveyard without losing you on card advantage (yay cycling). I have elected to run this card over Mishra's Bauble which accomplishes the same objective in a similar manner. The reason for this was, at first, due to budgetary reasons (iconic masters was not released yet and the bauble was going for upwards of $20 apiece). Now as I have played with this deck more I have found out that because Architects of Will is an Artifact Creature I can enable delirium easier. It can even be used as a pseudo combat trick by cycling before damage is dealt to pump Grim Flayer or Tarmogoyf. Another small plus is that he can be hardcast for a creature/fix the top three cards in a pinch. I run three to ensure I see one in a game but not to be flooded with them.
Bitterblossom is a card I am still playtesting with. It does put me on a clock but (this is the burn player in me talking) unmolested I can kill you faster with the tokens it generates each turn or they are free chump blockers/deterrents preventing larger life losses. It can also be my enchantment and tribal source for delirium if it gets destroyed or I pitch one to the yard.
Traverse the Ulvenwald is a one mana tutor when delirium is online. Being able to grab a creature or land of choice is powerful, especially games 2 and 3 where i have sided in some answer creatures. I only run two because it is a card i want to see once delirium is online and ideally not before. If i cant get delirium online it is still useful for grabbing a basic but it is not the ideal use for it.
Inquisition of Kozilek and Thoughtseize are my hand disruption spells that I am currently running as a 3/2 split. Pretty self explainable imho; I am running a BGx deck afterall.
Maelstrom Pulse is a singleton kill spell that is exceptionally versatile. While sorcery speed and three mana it hits any non-land permanent and sometimes can kill multiple permanents.
Liliana, the Last Hope is my primary planeswalker choice. I run two and elected her over Liliana of the Veil as my primary walker because her +1 ability helps push through the larger blockers (opposing Tarmogoyf, Tasigur, the Golden Fang, etc.) or kills tokens and more importantly her -2 ability is fantastic for recurring creatures to maintain boardstate. Her ultimate is not an ability I ever really expect to use but it isn't something I would be upset for using either.
Liliana of the Veil I run a one of because she is still a fantastic planeswalker and I am never sad to see her. She works great with the midrange formula. Her +1 ability is hand disruption and her -2 ability is creature removal that is difficult to get around and finally the threat of her ultimate is something that is always looming over your opponents shoulder. I also dont want three LtLH.
Collective Brutality is a phenomenally versatile card. It attacks spell decks, it attacks creature decks, and it can also provide a 4 life point swing for those races as well. There is a reason this card shows up in most decks with black mana. The discard effect for escalation is even less of a "cost" for us since not only can we just discard a dead card in hand but we can also sculpt our graveyard to help get delirium online as well. I currently run two mainboard but as long as I have two in the seventy five I will be happy.
Fatal Push is the "new" great kill spell for black decks. Being able to kill most creatures in the format for one mana is efficiency unparalleled.
Hissing Quagmire is included to provide deterrence from attack when my creatures are tapped out and additional late game reach in the event of a stalemated board. I run two for an increased, but not excessive, draw and in case one is killed.
Ghost Quarter is included to deal with troublesome lands. I only run one to minimize the chance it is in my opening hand (it is hard with a three color deck to have a colorless land in your opener) and because I can easily fetch it with a Traverse.