doctorhydrogen

Druids' Repository generates stupid amounts of mana in Prossh, and it's a cheap card.

manabasecrafter.com is a fantastic site to see what lands/rocks work in your particular color scheme.

I'm not a fan of Vivid lands. They enter tapped, and stop being duals after a few uses. I'd much rather run something like Darkwater Catacombs or Tainted Isle in those slots.

Tolaria West lets you tutor up Cabal Coffers with sucking up a deck slot.

If you're looking for a slot for Black Sun's Zenith, pull Unholy Strength. It's not going to win games, and it's just asking to get two-for-one'ed when someone kills the creature.

If you're just using Altar of Dementia for repeatable sacc outlet I'd run Ashnod's Altar over Altar of Dementia. Outside of a dedicated mill strat there's just too many decks that actually WANT stuff dumped into the graveyard en mass. Why help your opponents out when you could get mana off the sacc?

I figure if a deck can use Martyr's Bond and Grave Pact it can use Dictate of Erebos, too.

The one extra mana on Hero's Downfall to be able to hit artifact creatures AND planeswalkers seems worth it over Go for the Throat.

Similarly Fate Forgotten costs one more than Disenchant and it's an exile effect. At least where I play it's worth the one mana to be able to handle Theros gods, Darksteel Plates and Forges, Blightsteel Colossus, ignore enter-the-graveyard triggers, prevent Wurmcoil engine tokens, prevent someone from recurring the artifact, etc. Revoke Existence costs the same as Disenchant to exile too, but it's sorcery speed.

Vault of the Archangel does great work in B/W decks.

Now that commander tuck isn't a thing Dissipate is probably better than Hinder in 99.9% of situations.

I like Darksteel Mutation over Pacifism. Pacifism lets the dude still use abilities and passives, where as Mutation shuts everything down. And you don't care if it's a 0/1 indestructible blocker since your guys almost all have flying. Plus it sticks their commander to the board in a way that's hard for them to get it killed and brought back to the command zone.

You need at least some removal. IMO whenever possible go with exile removal as it deals with indestructible stuff, avoids graveyard triggers, and prevents graveyard recursion, particularly because a simple Leyline of the Void or Rest in Peace is going to shut down your commander if you can't deal with it.

I'd start with Utter End and Oblation as they hit anything, Return to Dust for artifacts and enchantments, and Swords to Plowshares, Path to Exile, and/or Unmake for dudes. A few board wipes are generally mandatory too for times when you're behind. Merciless Eviction and Austere command are favorites of mine in B/W.

I'm guessing Sylvan Scrying is there to get Gaea's Cradle? If that's the case Harrow would get it and put it straight into play.

With that many creatures Lurking Predators would do sick work.

In this build Primal Surge could easily put 50+ permanents onto the field, and since you're already running Concordant Crossroads there' a decent chance they'd all have haste.

I also had good luck with Everlasting Torment in Lydolda too since it more or less lets the damage stay on the creature through turns. I run a lot more tokens than you though so it might work better for me than you.

Darkest Hour is probably worth running just so you can draw a card off every sacc.

You could slot in Kher Keep pretty easily as a nice way to make a dude to sacc.

I'd stay with where you are for now just to see if you like Nath, then if you do start looking for slots for stuff like Dark Deal and Syphon Mind.

Thanks for the kind words about my feedback, too :) And I'll take a look at your immortality deck when I get a chance.

I'd pull Outland Colossus and Thousand Year Elixir for sure. If you want to keep the untap portion of Thousand Year Elixir you could replace a forest with Wirewood Lodge. I'd keep Genesis Wave; it's really solid. Allosaurus Rider seems expendible. Yeah, it's big but it's also expensive to cast and doesn't have trample.

You could also take a look at Squall Line or Windstorm over Hurricane because they're instants. Squall Line costs one more and Windstorm doesn't do damage to people, but being able to instant cast it at end of the person's turn or in an emergency seems worth it to me.

I clearly love Glissa http://www.mtgvault.com/doctorhydrogen/decks/glissa-the-traitor-edh-7/ so I wouldn't discourage anyone from playing her, but as the leader of an elf deck she doesn't synergize with other elves as well as Nath does, and this particular deck isn't taking much advantage of her artifact recursion.

On the other hand if you build Nath with too many discard effects it can be pretty unfun to play against. I personally try to build decks that, while strong, also lend themselves to fun and interesting games where even the losers had a good time, and blowing everyone's hand doesn't lend itself to that.

I'd maybe test Nath just as is, swap him into the command role and Glissa into the deck and see what happens. If you kinda wind up liking him more you could then sculpt the deck more to work with him if you want.

If you're gonna keep Glissa you could swap out Go for the Throat for Executioner's Capsule. It gives you one more permanent to hit with Primal Surge, but more importantly if you kill someone with Ex Cap with Glissa out it goes back to your hand immediately so you can play it again and do it again.

With that many permanents a Primal Surge could easily put 50 permanents into play.

Lurking Predators would do serious work in a deck with so many dudes.

Steely Resolve set to elves would give all your guys hexproof. Cover of Darkness set to elves would make them pretty hard to block.

If you want to run an enters-tapped dual I'd take the one life off Rugged Highlands over the does-nothing-gate-subtype of Gruul Guildgate.

Flamekin Village is a great haste enabler. Yavimaya Hollow is really useful in green, too.

I'd get a slot for Greater Good. Drawing ten gazillion cars off sacrificing Thromok to a board wipe or an attempt at stealing him seems great.

I'd run Deglamer or Unravel the Aether over Naturalize. Naturalize won't hit Theros gods, Darksteel stuff, Blightsteel Colossus, etc. Plus stuff sent to the graveyard with a Naturalize is a lot harder to get back than something tucked into the library.

I ran Vorosh +1/+1 counters for awhile (eventually switched to Mimeoplasm then Jenara). Mimeoplasm was a much more effective commander even without doing degenerate Mimeoplasm stuff.

Bred for the Hunt is cool flavor-wise but why worry about having counters on your dudes to draw when Coastal Piracy and Bident of Thassa will draw no matter what.

Realm Seekers usually comes into play with like a gazillion counters onto it, plus can be used to tutor lands.

Spike Weaver can save games.

Ob Nixilis the Fallen can be amazing, and if you run him I'd find a slot for Fertilid, but literally the best was in commander history to win a game is by forcing your opponent to search up lands with Fertilid with an Ob Nix in play. No joke, it's the greatest thing ever.

As far as counters-on-a-stick goes I'd rather run Draining Whelk over Mystic Snake. Sure it costs a bit more but it has evasion and comes with +1/+1 counters. There were games it flat out won games when I'd counter a Elesh Norn or something with a Corpsejack Menace or Doubling Season out and have a +14/+14 flier next turn.

I'll keep your budget in mind with my suggestions.

40 lands is probably more than you need for a mono-colored deck. You could shave down to 38 easily. I run 36 myself in monoblack.

You need more draw. Kothophed, Soul Hoarder is worth the risk imo. Promise of Power is great draw AND it makes a demon so it's on theme. Damnable Pact can do good work, and I've used it as a finisher before.

Hero's Downfall will probably be a dollar card once it rotates next month, but even now it's only three. I'd replace Doomblade with it, it's so much more versatile for one more mana.

I'd run Murder over Flesh to Dust.

Whip of Erebos is gonna gain a lot more life than Staff of the Death Magus, and it has added utility.

33 land feels light.

I generally think Vandalblast is better than Shattering Spree for multiple artifacts. Sure there are times Shattering Spree is cheaper, etc. but late game an overloaded Vandalblast can take out a dozen artifacts for half the cost Spree could do it, and more importantly Vandalblast doesn't target stuff (when overloaded) which can be invaluable.

If you're just looking to dump apostles quickly and get a big dude out you could run stuff like Dark Ritual to get as many out asap as you can.

Thrumming Stone does good work