So, I feel like I've hit a good spot with the Peer into the Abyss combo to post a more functional list and explain a few of the choices I've made for certain cards. I'll do a list format to make it a bit easier to read through. Feel free to leave comments and suggestions. I'm always looking for input on builds like this.
Creatures-
Lurrus of the Dream Den: This is a fantastic value card with an enchantment removal package. Bringing back a Dead Weight or Mire's Grasp every turn against aggro is amazing and can really break your opponents resolve. Especially if they don't have removal for the cat. 2 health is a bit on the low side, however, and he dies pretty easily to most removal. Side him and the enchantment package out against combo and control and bring in your discard options.
Murderous Rider: A 2/3 lifelink body stapled to a top tier removal spell is, quite frankly, amazing. Hitting planeswalkers is especially good.
Palladium Myr: Needed ramp. I'd prefer he cost 1 less, but I don't make the rules. He's needed for non-green ramp and also makes a serviceable chump block if you absolutely need him to do so.
Solemn Simulacrum: Sad Robot is also a necessity for ramping without green. 4 mana for a 2/2 isn't super awesome, but the land tutor and card draw more than makes up for it.
Enchantments-
Dead Weight: Really good early game removal hitting almost every single viable 1 mana creature and a decent amount of 2 mana dudes they can drop as well. Bringing this back turn after turn with Lurrus feels really good too.
Mire's Grasp: Almost exactly like Dead Weight obviously.
Underworld Dreams: Half of the combo. I see most people running this as a 4-of for the combo, but I prefer the 3-of here and a 2-of Ob Nixilis. The slower tempo we're currently seeing gives a couple extra turns to draw into this, and with all the scry effects, it's not terribly difficult to find.
Artifacts-
Mazemind Tome: One of my absolute favorite cards from M21. Being able to Scry up to 4 times per tome at instant speed is really, really good. The fact that you can draw as well AND gain 4 life after it's used up makes it absolutely essential for this deck. Thus the 4-of.
Bolas's Citadel: Stay with me here... This is amazing with Scry effects, and protects your combo pieces by letting you cast them off the top of your deck. A surprise tapped out Murderous Rider can be absolutely back breaking, even if you have to pay 5 life for it.
Sorceries-
Peer into the Abyss: Our win con. It costs a lot, and can be difficult to get to sometimes, but the deck has been surprisingly resilient for mono-black and this goes off far more than it seems like it should based on the casting cost. Scry effects are also good at pulling this up for you (especially since I don't like Grim Tutor in this list).
Extinction Event: If you're on this website I'm assuming you know that situational sweepers are pretty good. Not as good as full board clears, but this will help you survive an onslaught of creatures.
Planeswalkers-
Ob Nixilis, the Hate-Twisted: He acts as the 4th and 5th copy of Underworld Dreams, as well as being hard removal. Letting your opponent draw cards isn't always super awesome, but if you get a couple Underworld Dreams effects out it'll add up pretty quickly. I really like him in this build. You could always run a 4th copy of Underworld Dreams and bring in Grim Tutor instead, but he just seems more impactful to me.
Ugin, the Spirit Dragon: Essentially a sweeper in this build, although the 3 damage can be helpful as well. The ultimate is essentially a win-more for you. As a bit of a bonus, I've seen a lot of players just scoop when he hits the board. An impactful top end of the curve is always nice.
Mana Base-
For anyone that doesn't know me, building functional mana bases is my absolute favorite part of MTG. And talking about how I choose which cards to include is also pretty high up on my list of things I enjoy.
23 lands may seem a bit low for a combo/control list, but by including tutor effects (Sad Robot and Fabled Passage), mana dorks and Scry effects, I get a hell of a lot of mileage out of the deck and can run a couple extra cards to handle different scenarios. Topping out at 4 CMC for the majority of the deck also helps since stalling at 4 or 5 mana for a few turns doesn't necessarily turn the deck off since there's usually something you can do. Scry lands may seem awkward, but the format is currently slow and they help set up future draws or Bolas's Citadel. I tend to treat additional effects as either 1/4th or 1/3rd of a land drop, so I see it as essentially running around 26 lands in the deck while still getting a bit extra gas when and where I need it thanks to situational card draw.
If you've read this far, thanks! I also hope that if you decide to import the deck that it performs decently well for you. I'm under no delusions that this is anywhere near a tier 1 or 2 list, but it can act as a surprise rogue deck and it feels REALLY good to see all the triggers pop up on a Peer into the Abyss.
