Let's start by talking about the weaknesses of the Mono Black - Coffers Shell:
- the mana is clunky, and can make you stumble when it is most critical
- playable black cards usually consist of discard and removal, limiting your
interactions
- most of the cards mentioned above trade 1 for 1, and have low card quality
compared to other modern staples, thus resulting in bad topdecks, and dead
cards in hand
- being flooded with high cmc cards you can not play, because you are starved on
lands
The current staples in a Coffers deck are:
- various low cmc removal and discard to slow the opponent like Fatal Push,
Sheoldred's Edict, Thoughtseize, Inquisition of Kozilek
- March of Wretched Sorrow is a versatile removal and enables valuable life gain, but is slow and really shines later in the game
- Cling to Dust and Shadow of Doubt to answer the combo meta
- Field of Ruin to answer Domain, Tron, and sometimes Amuelet Titan
- Damnation or Deadly Cover Up to reset the board, the latter even cripples combo
decks, since it has a Surgical Extraction effect
- The One Ring to stabilize and find your anwers and threats
- Sheoldred, the Apocalypse. She stabilizes and is a threat
- Karn, the Great Creator as your swiss knife, and wincon
Many believe, that Karn, the Great Creator belongs in Cabal Coffers as naturally, as UW-control decks will go into turns.
The first thing you have to realize is, that you have no need to play Karn!
It may be quite the strength of this deck to utilize Karn and his sideboard, but it is also a considerable weakness.
And I believe, it is a trap thinking you need him, to get the most value out of the mana you produce with Coffers. You do not need him to have the answer to every possible situation either. As versatile as he looks at a first glance, as limiting he reveals himself to be, if you have a need for specialized answers against that one dominating deck in your meta. And him being a threat is kind of optimistic, because he himself usually is not the threat. You take one turn, and 4 mana, to SEARCH for your threat, that you then play your next turn. That is not quite the powerlevel we are looking for in modern. He is still viable, by all means, he just is not as glued to the deck archetype as you might think he is ;)
The second thing is realizing the diminishing value of modern staples like Thoughtseize and Fatal Push in this shell. Those cards trade 1 for 1, have limiting factors like lifeloss, or cmc restrictions, and are horrible cards to draw later on.
We are not a typical midrange deck, in which those cards are better suited.
The third thing to realize is, that you are not limited to being control deck that comboes off of Cabal Coffers.This shell is, by nature, a control deck, because of the cards used above, and the nature of Mono Black interaction. But there are Mono Black Midrange decks out there, playing the same, and some different black modern staples in form of creatures, while maintaining the control aspect, and without the need to rely on some Cabal Coffers payoff. They have a stronger mid game, lack the strong late game Coffer-Decks has, but are still doing fine.
When Cabal Coffers joined the Modern-Meta people played it as a combo-control deck mainly. Cards like Expedition Map and Profane Tutor were staples for quite some time, and are even played up to this day. The goal is to assemble Coffers and Urborg as fast as possible, to then dominate the board with Ulamog and Wurmcoil Engine... wait, I am mixing something up here... In all earnest, the combo-variants tried to be some kind of imitation of Tron, combining the high cmc - high value cards with the mana ramp Cabal Coffers offers. But it just is not that kind of deck, at least in my opinion. Yes, it is important, and an advantage to generate mana ahead of curve. But the deck does not have enough enablers to be reliable in that way. Look at Tron or Amulet Titan, their enablers, and the mana they can produce. Just looking at the amount of mana available at certain turns, and the reliability in doing so, those decks outperform the Coffers-shell significantly.
If you built it up to be a reanimator-combo, relying less on the mana, and more on the reanimator shell, you too lack enablers, reliability, and the ability to recover if your combo failed. Esper Goryo's-Reanimator shows you how it is done, and what we lack.
The One Ring, as a common stabilizer in slower, and combo decks, does not compensate enough for the many bad draws you have, until you can combo again. That may be the reason that the reanimator version of coffers is the least played among its variations.
Enough of bashing my favourite deck! Let us talk about its strength for a change of tone.
First of all, the backbone of the deck.
It is not Cabal Coffers... I know, I know, the name of the deck suggests otherwise. But, think about it. In terms of mana production we already mentioned decks that outperform us. So what are the cards that generate the most value, and are welcome in almost every boardstate? Well, in my case, I am always relieved to see The One Ring and Sheoldred, The Apocalypse. Especially in combination, they generate insane value, stabilize your game through protection, card draw, as well as lifegain and a thicc body to protect you. Sheoldred is enough of a threat that opponents want her to be removed within their next turn at the latest or it already feels like a loss to them. In my eyes, she is compareable to Omnath. You just do not want to let your opponent untap with her in play! In my opinion you need a playset of both, to have them reliable in play. Many versions cut Sheoldred by up to two copies. That won't be possible with my deck strategy, you will realize why later on.
So far, nothing new. I guess Coffer gamers will agree with the statements about those two cards, since after their arrival the deck seem to have risen in popularity.
So what do I do different?
I have chosen different cards to interact with my opponent than the usual Fatal Push and Thoughtseize. Nothing you have not heard of yet. It is more like a different perspective on cards we all already now. Let me ask you this:
What is the important difference between those two cards, and March of Wretched Sorrow? They cost only one mana, and have a solid, cost efficient effect for the mana you pay for it. They are the best you can get for that cmc, while March seems very awkward and underperforming at the same time. March shines later in the game, after you got enough mana to fuel the bottomless pit that is lif... I mean the X in it's mana cost. But wait... there is more text on that card. You can exile another black card to pay for 2 mana in its X cost!? That is something those other two do not have. Seems rather unreasenable though. Why would you accept card disadvantage, if you can just play something more costefficient? It is not like you can draw tons of cards later on to make up for it... ohhhh, wait, we can! Suddenly that early game card disadvantage does not seem so bad anymore. It is still not "good" by any means. But it enables us to play that card ahead of curve, even if we are mana screwed by having drawn the second Coffers, instead of the first Urborg AGAIN. So being able to cast this "free spell" even though my mana is unreliable makes that spell kinda good, I guess!? It is our unique Coffers-shell, that lifts this card on an equel level as long term modern staples like Fatal Push and Thoughtseize. That mechanic of paying a different casting cost offers some much needed flexiblity. On the topic of "free spells"... I heard that term before, didn't I? There is Grief. It is a Thoughtseize, that does not cost me two life, and later in the game, it even brings a 3/2 body on the board! <- You see my perspective? At any point of the game it is a Thoughtseize for zero mana, without lifeloss, and one black card as its cost. And if you have the mana for it, it still is the before mentioned, PLUS a 3/2 menace threat. Why did I even play Thoughtseize to begin with!? And there we have Force of Despair. I have never seen this card in modern... I wonder why? It is a Fatal Push I can play at turn ZERO, without any cmc restritctions. It even kills a, Scion of Draco, Primeval Titan and Murktide! And there is even more! Living End? Dredge? Rhinos? Elves? Yawgmoth? Domain Zoo? All those decks that play multiple creatures in one turn... It even acts as a boardwipe at instant speed! All of that for one black card as its cost. Seems unbalanced, doesn't it? So what is the drawback. Well, the spell is only effective in the very turn those creatures entered the battlefield, and further more... no, wait...that's it...there is no more drawback...
Those spells are considered bad because of the card disadvantage. That is, of course, reasonable. And you are right, in a fair fight, with fair cards played on curve, Fatal Push and Thoughtseize are the allstars of black cards in the meta. But that is not quite the modern meta I know. The powerlevel of cards has continously risen, and it has long been established, that those "free spells" and their card disadvantage is not only acceptable, but often even considered broken in powerlevel to the point where people demand them to be banned without even thinking twice about it.
Let's face it. Fatal Push and Thoughtseize are not good enough any more. Grief and Force of Despair seem to be comparable weaker in the early game, but can STILL be played at every instance the before mentioned can, without paying mana for it, and offering the same, if not more effect PLUS offering even more value later in the game, if you can pay the usual mana cost. Sure, Push seems to be better against a Kavu turn 2, and Force of Despair even a waste against Ragavan turn 1. But Force of Despair can not only remove said Kavu and Ragavan, it can remove that Scion of Draco, AFTER you have already tapped out to play your Dauthi Voidwalker. Chef's Kiss, I tell'ya.
Now that we have established The Ring, Sheoldred, March, Grief, and Force of Despair as our key cards. How do we fill the remaining 16 slots?
I guess it is fair to say, that Dauthi Voidwalker is a given. He offers disruption, as well as a threat. That is exactly what we need! Cards that do multiple things at different stages of the game, or all at once. And all the before mentioned cards do exactly that. 12 Slots left.
1-2 Cling to Dust never seem wrong. The card does not present much value most of the time, but can prove to be the droplet that tilts the scales in your favour.
2-3 Damnation, or Deadly Cover-Up is at least card advantage in a way. Maybe substitute them comletely, if Force of Despair turns out to be enough for your meta.
1-2 Sheoldred's edict should probably be in there. The card seems to be way to bad, I mean fair, but it does not target and can remove a Scion, Tefery, or Karn.
It definitly has its uses, but is also very lacking, since your opponent can easily play around it if we need to remove that Scion, Titan, or Murktide.
1-2 Invoke Despair do seem to fit in there too. They usually do two things at once, being removal and drawing cards. Doing multiple things at once was deemed good so far, but they are kind of awkward in their manacost. I really like that card, but it usually feels lacking, and only sometimes presents what we need to turn a game around. They are especially good against Leylines and Dracos, if that somehow matters...
1-4 Profane Tutor are a useful tutor, either to fix our mana, or get us that removal we need. It is slow, and ideally you want to play it turn 2 to really matter.
You might want to interchange any card with a Profane Tutor, just to get the playset in your deck.
4-6 slots left. Now it gets difficult to choose. We already have discard, removal, multiple threats, boardwipes, card draw, life gain, graveyard hate...
I believe the remaining slots are quite flexible, and can be adjusted to your personal needs. If you play in a combo heavy meta, play Shadow of Doubt and Thoughtseize.
Maybe you just can not leave Karn behind. There you are, he still fits right in.
I myself am a sucker for Liliana of the Veil. Her powerlevel may not be quite up to standards anymore, but at this point logic does not matter any more, she deserves to be a modern staple once again! I firmly believe that! And apparently she is good against Dracos, or so I heard... Her spot will probably be taken by a new card in a couple of weeks.
Insatiable Avarice is that very card, and it is insane for this deck. It does multiple things at once or at different stages of the game (remember, thats the power level we need), it is a tutor for ANY card, but that is not the good part, we already got this, then it draws 3 cards and you loose 3 life, on top top of tutoring a card! Oh, and you might want to target your opponent with it, if you have a Sheoldred in play, to drain them for 9. So it even works as a finisher.
My first instinct was to swap Liliana and Insatiable Avarice, since I played a version without Profane Tutor for some time. If you play Profane Tutor, you just swap them with Insatiable Avarice in my opinion. Concerning Liliana, I really am fond of her, but most of the time she amounts to an edict effect for 3 mana. She is a threat though, and your opponent probably wants to deal with her in some kind of way. I guess, she is in a spot, in which you might want to play different cards, according to you individual meta.
1-2 copys of Griselbrand are a good fit too. He synergizes with Sheoldred, and is a real powerhouse to the point he is still a common reanimator target in other shells.
Some might want to play a Troll of Khazad-Dûm, since he offers something in the early game in form of mana-fixing. With my strategy of pitching cards into Grief, Force, and March, we usually do not have "dead" cards in hand. The decision between both of them just comes down to your preferences. Do you want to be more reliable in the early game through mana-fixing, or do you want to have a better threat and rebound through card draw in the late game?
Was there anything else? Oh yes, the manabase...
Just play 3-4 Coffers, Urborgs, Citadels and Field of Ruins.
1-2 Castles and Takenumas, and
fill the rest up with basic swamps. I believe that there is only a miniscule statistical argument for playing one copy more or less of each.
If you are fond of the field-effects you can add Demolition Field in the mix.
In my experience it makes the manabase even more clunky and unreliable. Already having 8 colourless lands usually seems to be OK, but challenging Klothys by upping the colourless mana even more seems rather daring.
After all that, why even bothering playing Cabal Coffers, you might ask. Is there even a need having that insane mana engine, if you do not use it by playing cards with 6+ manavalue!?
No one said to not play cards with 6+ mana value... But does the manavalue necessarily have to be in one card!?
The strength of this deck is playing strong cards in the early game (at the cost of card disadvantage), recouping that disadvantage in the midgame, by playing the same cards without disadvantage, and drawing new ones with The One Ring, to finally overwhelm your opponent in the late game. What can they do if you empty your 7+ card hand, play 6+ manavalue in your turn AND still play some free spells while being (almost) tapped out!? Of course that is how it goes... Every game...Does it not? Wait, was that a dream, too?