doctorhydrogen

IMO any deck with this many big, scary beaters needs to be running Homeward Path.

You need more draw or you run out of gas in Atarka:

Greater Good: #1 priority here. You don't necessarily use it for draw so much as to turn removal into a win. A Wrath of God or targeted Doom Blade to a dragon is a lot less annoying when you can net four cards out of the deal.

Hunter's Insight: It's probably going to draw you a minimum of 5 cards, and if Atarka is out it will get you 10+. It's a star in this deck.

I'd start there and at least consider Sylvan Library, and Lurking Predators could easily put 2-3 dudes onto the battlefield per round in a multi-player game.

Kessig Wolf Run can turn tides quickly, and there's little to now downside to running it. I'd find room for Homeward Path, too . . . nothing sucks more than people stealing your guys and whacking you with them.

Dense Foiliage is fine, but I'd rather not protect everyone's creatures when I can just protect my own with Steely Resolve set to dragons.

It's largely meta-dependent, but if you've got a lot of decks packing fliers (which my meta does) when Bower Passage essentially makes all your dragons unblockable excepting for the occasional thing with reach. Just something to consider if you find yourself squaring off against bird tribal and Bruna Light of Alabaster decks often or something.

IMO Cabal Coffers is just to absurdly good in mono-black to not run Vesuva and Thespian's Stage to copy it and Expedition Map to find Coffers or a copy land if you don't draw them. I've never been like "F*ck yes, glad I topdecked that Leechridden Swamp or Barren Moor!'" but I've certainly giggled gleefully pulling a way to copy my already-OP Coffers.

Black Market is pretty absurd in a Shadowborn deck, too.

IMO short of a Nekusar deck I'm not a fan of Seizan. The last thing I want to do is give out six free cards to the table for every two I draw . . . assuming I even get to draw them. Too often when I ran him someone else just takes the free Sign in Blood off him before killing him and I wind up getting nothing out of the deal.

I ran Anowon as the general in my vamp tribal deck for awhile and at least in my meta he just didn't work. I switched to Drana and the change was night and day. For starters, Anowon doesn't impact the board at all until he survives a full turn, and nobody lets that happen because they can see he's coming down and someone always holds a removal spell because he's going to affect everyone. Drana can come down and be Greaved or Booted and one-shot someone. Plus people don't always hit her with removal even if they have it because they want to let you beat one someone else first. I'm not saying you should switch immediately, but next time you play the deck just swap the two for a couple of games and see how it works. I think you'll have better luck with Drana.

Cabal Coffers is absurdly good in mono black. Why not copy it with Vesuva and Thespian's Stage?

Hero's Downfall has the same CMC as Murder and can also hit Planeswalkers. I'd run it over any of your three removal spells.

Ascendant Evincar is really great. I'd find a slot for him. Vampire Hexmage is really solid, too. She's a decent quick drop early, but her real power is being able to kill a planeswalker on demand is really useful.

Training Grounds is going to make Roon's ability cost nothing to use.

That's like saying I sent the redhead home because the blonde is slightly hotter. Two is better than one, especially when the one is that good :)

Then again, I have no idea what I'd cut to make room.

Infernal Darkness can flat out end games in mono-black. Even if you get it out early it can function as a near time-walk, making people do nothing for a couple of turns. It's almost always great.

Druid's Repository is a monster in Prossh. Attacking with the tokens you were gonna immediately sacc anyway gives you 6 (or 8, 10, etc) counters on it. It's stupidly good.

Sundering Titan is banned.

Maybe Black Sun's Zenith over Infest? Yeah, you have to spend 4 instead of 3 to give dudes -2/-2, but they're permanent counters so they'll weaken stuff you wouldn't kill, and it has the option of pumping more mana into it if need be. IMO the flexibility is worth the slot, and after the reprint in Commander 2015 it's now a dollar card, qualifying it as pretty close to a bargain.

Crackdown is fantastic in mono-white decks.

Emeria, the Sky Ruin would be good here since the threshold is fairly easy to hit.

Rather than Path to Exile or Contemn or Swords, maybe run stuff like Act of Treason or Mark of Mutiny? May as fling the dude you're gonna kill rather than kill it and give them life or a land.

Illusionists's Bracers is good here, too. More than once I've copied a Serra Avatar fling with it.

Chromatic Lantern is strictly better than Obelisk of Esper, and the mana fixing it brings is AMAZING in a three color deck, even one with as many colorless spells as this one. Coalition Relic would be better, too, but I'd run Chromatic Lantern first.

I've had great success with Exotic Orchard in my Esper deck. Someone almost always has a least two colors I can use if not all three, and all it takes is one person running a City of Brass, Mana Confluence, Vivid land, etc to have it be able to produce anything. Seriously, try it, it's really underrated, and won't be nearly the tempo hit as something like the Vivids, Terramorphic Expanse, etc.

Strip Mine is under five bucks and is way better than Ghost Quarter in almost all situations.

Personally I'd run Expedition Map over Armillary Sphere since it lets you fetch non-basics, which can be invaluable.

Quicksilver Fountain and Back to Basics would flat out ruin some people's day going against this deck.

Gauntlet of Power, Caged Sun and Extraplanar Lens will probably do more work for you than Thran Dynamo here.

There aren't a lot of situations where I'd rather run Psionic Blast over Spin into Myth or Reality Shift for creature removal, though Psi Blast certainly is worth tons of style points.

Negate is a great spell in a lot of formats, but I'm not sure it's worth saving one mana casting it over Dissolve, Dissipate, Hinder or Spell Crumple and the bonuses they bring if it means you can't use it to deal with an Elesh Norn, Avacyn, Prossh, etc. I'd much rather pay the extra mana knowing I can deal with any threat, and get a bonus scry or exile or tuck effect on top of it.

Not much downside to running Valakut, the Molten Pinnacle, and a lot of upside. Nykthos, Shrine to Nyx would probably generate you a ton of mana in this deck, too.

Mana doublers? Caged Sun, Gauntlet of Might (and Power if you're rich) and Extraplanar Lens would do a lot of work here.

I honestly have the same problem in my mono-black deck, finding room. As it stands now I can't slot in Bubbling Muck or Infernal Darkness, both of which I'd love.

I'd say cut some swamps. 47 is a lot of lands for a multi-colored deck, let alone a mono-colored. It's going to land-flood you too often I think. You could easily cut down to 42 or even 40.

I'm rather of the same mind regarding Dolmen Gate. You can slot it in any deck and have it do solid work, so I tend to only run it in a deck where it serves a purpose beyond just being an any-situation all-star, but Brimaz would seem to qualify as a legit usage.

Battle Mastery wins games in Sigarda decks.

Primal Surge would be great here. In a deck with only 86 instants and sorcery spells it could very easily put 30+ cards into play.

Dolmen Gate lets you attack every single turn regardless of whether or not it would normally kill Brimaz and the token he makes.

You also have a little breathing room to swap in some utility lands. Flamekin Village, Kor Haven, Slayers' Stronghold and Sunhome Fortress of the Legion are all pretty decent and wouldn't hurt your mana base too badly.

Recurring Nightmare is banned in EDH.

Very rarely, if ever, is Empty the Pits going to do more work than Rise of the Dark Realms.

Vesuva and Thespian's Stage give you another copy of Coffers. Deserted Temple untaps them. Nykthos, Shrine to Nyx would also kick a hell of a lot of mana in this deck, too. Expedition Map would fetch Coffers or any of the above.

I'd find some room for some doublers. Caged Sun, Extraplanar Lens, Gauntlet of Power, etc. do serious work in mono black.