Mono green devotion combo deck for competitive modern. Updated 8/6/18.
I have been playing and tweaking this deck for about nine months now and I found it fits into the current meta very well. It is fast enough to beat out other combo decks regularly (storm, KCI), consistent enough to survive fair decks and big mana decks (tron, humans, affinity, burn) and resilient enough to sneak wins past hard control decks (UWx). The most difficult matchups are generally jund and mardu pyromancer where they pack both a lot of hand hate and cheap removal, making you rely a lot more on your cycling creatures and top deck.
Primer
The point of this deck is to present incredibly fast and reliable ramp and board development in order to play lethal combo within the first few turns or lock down your opponent long enough to grind out a win. At its core this is a combo deck that relies on creature pairs to go from nonthreatening to victory in a single turn. Everything in the deck is built around drawing, protecting and ramping to be able to play out the combos as early as turn three. The sideboard for this deck has been in testing and flexible and the red splash has been very effective at improving the deck's game two/three against many meta decks.
Playing the Deck
Early Game
Your ideal hand has ramp on at least turn 1 (hopefully turn 2 as well), solid early devotion and at least one payoff card or piece of interaction. You want to play Arbor Elves turn 1 almost without exception. With a turn 2 Utopia Sprawl you will then have 4 mana to spend on something rewarding like a Garruk Wildspeaker or one of your draw/devotion creatures or additional mana dorks: Elvish Visionary, Carven Caryatid, Wistful Selkie or more Arbor Elves or some Birds of Paradise or even additional Utopia Sprawls if you have them, though those should have gone out before untapping the land with the elf. This means that by turn 3, you should have most of your engine on and can start playing your payoff cards or win the game, depending on your draws. Nykthos, Shrine to Nyx acts as a colored source with a devotion of 3 and as ramp with a devotion of 4+ so playing it before turn 3 generally wont help. Leylines of Vitality replaced Burning-Tree Emissary recently due to having the same devotion, regularly being just as free and having two abilities that are surprisingly relevant to the board/game state.
Rev the Engine
Now that you have devotion and access to a lot of mana, it's time to go to work. The deck's payoff cards are used to grind out powerful value, control your opponents or simply win the game. They start at 4cmc.
The first is Garruk Wildspeaker. Garruk provides 2 devotion and if you have a Utopia Sprawl forest and a Nykthos, Shrine to Nyx out, he is effectively free to cast. His ability to untap two lands and the fact that most of our important lands should be producing multiple mana each is one of the fastest ways to ramp into Tooth and Nail or begin a Primal Command soft lock. If for some reason you don't have anything to do with your mana, he can create 3/3s to generate board pressure or act as blockers or sit above 4 loyalty and threaten an overrun kill using his ultimate ability and your mass of devotion creatures and dorks as an alternate wincon. Always activate him for this reason, even if you are just untapping two or your already untapped lands.
The second is Nissa, Vital Force. Like Garruk, she can untap your lands that produce mana though with the risk of animating them and making them vulnerable to removal. Usually I will animate one of my unenchanted lands to represent a blocker if I need to plus her unless I'm immediately using that land for mana. Her minus ability can get back permanents to your hand which can greatly improve the deck's longevity and let you grind out additional value against removal- and control-heavy opponents. Her ultimate is another good reason to play her as the turn after she comes out she can be made into an emblem so that all of your lands played draw you cards, limiting the ability to flood. Finally, she provides 2 devotion. She is more of a grindy card than the deck probably wants but she's one of the best ways to sustain yourself in a grindy match where you can't combo properly.
The third, also at 5cmc, is Primal Command. This card is the butter to the rest of the deck's bread. Every single mode is useful and it can be used to help you develop your board state while suppressing your opponents and getting to where you can combo kill them. The first mode is good for buying time in races (like against burn) and killing Death's Shadow. 7 life is nothing to sniff at, especially when paired with another mode. The second mode is arguably the most powerful, letting you chain Primal Commands to soft lock an opponent by putting a land or other important noncreature permanent back on top of their library so they don't draw additional gas/lands when they need them. It is soft removal and buys you time while the other modes get you the action you need. The third mode is the least powerful but not irrelevant. You can restore your graveyard to your deck to bring back threats or pieces you need or shuffle an opponent's graveyard away, hurting decks like Grixis Shadow, Storm, Dredge, Vengevine, etc. Note that this happens before the fourth mode if you choose both so you can shuffle your graveyard back in and then tutor for a creature that was previously in your graveyard. The last mode is one of the most common modes to use, especially in combination with the second mode. You can tutor for Carven Caryatid if you need a blocker and devotion, Wistful Selkie if you need pure devotion, Arbor Elf if you have enchanted lands but no Nykthos, Shrine to Nyx for more mana or most commonly, Eternal Witness. Using Primal Command to put something on top of your opponent's deck and get an Eternal Witness which you can then play and get back your Primal Command (or other payoff spell) is one of the most common things for the deck to do. It allows you to develop your board state while suppressing your opponent. With enough mana or a Nykthos, Shrine to Nyx out, it is very easy to chain doing this multiple times in a turn or multiple turns in a row. You can often cap it off by fetching and hardcasting Emrakul, the Aeons Torn with all of the board development you have been doing while denying your opponent the same.
Combos
Emrakul/Xenagos
How it Works
Emrakul has protection from colored spells but that doesn't prevent Xenagos from targeting her with his ability. You simply go to combat, target Emrakul with the trigger and swing with a 30/30 flying trampler that happens to have annihilator 6. If for some reason this isn't lethal, repeat as needed.
Issues
Emrakul is notoriously difficult to kill but given the nature of the deck, there is a very good chance Xenagos will be a creature when you pass priority on your first main phase. While he is indestructible, he can be hit with a Path to Exile or Dismember leaving you with a slower and sadder Emrakul that is unable to attack this turn. Emrakul also has to attack for this to be effective, meaning cards like Ensnaring Bridge or the tap mode of Cryptic Command can both pause this at the very least.
Notes
If Xenagos is removed at the end of your main phase, you still have an Emrakul on board and are threatening a very similar and scary move the following turn. Barring a board wipe, getting her off the board is surprisingly difficult. Given the nature of the deck, though, an opponent boarding in a board wipe or U/W control having main board board wipes isn't out of the question. Also given the nature of the deck, it is entirely possible to hardcast both pieces of the combo in the same turn if needed, or to cast them independently as they are both threatening on their own.
Kiki Conscripts
How it Works
One of the many infinite Kiki-Jiki, Mirror Breaker infinite combos, this works by using the ETB ability of Zealous Conscripts to untap your Kiki, which you use to create a Conscripts, which untaps Kiki, etc. You create an arbitrarily large number of Zealous Conscripts tokens and swing at your opponent for an arbitrarily large amount of damage. Very simple. Also, having out a Leyline of Vitality means you will gain infinite life even if you don't swing.
Issues
This combo is a lot more fragile than Emrakul+Xenagos. It relies on two unprotected creatures that can be killed with most conventional removal (though not Fatal Push) and it requires you to continue the loop until you have made a lethal number of attackers, then move to combat, then swing. Unlike Emrakul+Xenagos which both have protection (Emrakul's protection from colored spells and Xenagos's indestructible/sometimes not a creature) this can fail if your opponent has a number of different answers. It also doesn't get under most Ensnaring Bridges, another benefit Mike and Trike had.
Notes
This combo took the place of Mike and Trike for two reasons. First, both pieces of the combo are individually playable, useful and aren't required to have each other out to function properly. Borrowing an opponent's Karn and having it exile itself, cracking someone back with their Ulamog, or even just being a 3/3 body make Zealous Conscripts a useful card on its own. It can also untap one of your lands, though if you're generating enough mana to cast it easily you probably don't need to do it. Kiki on his own is absurdly powerful. Since nearly all of the other creatures in the deck have ETB effects or activated abilities, you can effectively tap Kiki to: Draw a card, return a card from your graveyard to your hand, untap a forest, add any color of mana to your mana pool, or simply generate a consequence-free blocker. Triple red will usually require two red sources (which includes Utopia Sprawl) or two untappers but if he sits on the board for 2-3 turns, you often will generate so much advantage that you wont even need to combo with him.
Optional Combo:
Mikaeus/Triskelion
How it Works
This is often the go-to combo for ending games. It allows you to win at instant speed and even if your opponent has removal, they need to know what to play and when in order to prevent you from going infinite in response. By having both creatures on the board, Triskelion can remove two counters from itself in order to shoot itself twice (as Mikaeus grants it base power and toughness 2/2) and then use the remaining counters on it to shoot your opponent. Then it dies and due to you removing all of the counters from it, it triggers undying and comes back with 4 counters. This can be done infinitely in order to do infinite targeted damage, winning the game.
Issues
Cards like Rest in Peace or other ways to prevent undying simply stop this combo in its tracks. However, if your opponent is actually playing graveyard hate against your deck that means they aren't playing something more threatening and that's a blessing. Targeted removal also can stop the combo but only if your opponent does it correctly. If you are going through the loop, they must get rid of Mikaeus in response to the undying trigger or you can continue to do it at instant speed and still win.
Notes
The first two counters you remove from Triskelion, even after it has come back with four counters the first time, always ping the Triskelion itself. By walking through the loop in this particular way, you force your opponent to know that they have to get rid of Mikaeus in response to the undying trigger. To explain: If an opponent tries to path either creature or kill Mikaeus in response to you removing the first counter, you simply remove two more both targeting itself causing it to die and then trigger undying, letting you win with the removal still on the stack. If they do it in response to removing the second counter, you target it with the third removal and it dies, undies and you win with it on the stack. If they do it in response to removing the last counter, Triskelion dies as a state-based effect (it has two damage marked on it and is now a 2/2) and triggers undying. This is the only time that Mikaeus is vulnerable. If you are presenting the combo, walk through each step and make your opponent decide when and how to use their removal (if they have any) so that they have multiple opportunities to misplay.
Why it was replaced
There are two simple reasons why Kiki Conscripts was put in for Mike and Trike. First, Leyline of Vitality proved to be a very powerful include and it nerfs Mike and Trike significantly. With one leyline out, Triskellion has to target itself with all three of the initial activations instead of dying to state-based effects with the third activation, meaning it is much easier to play around and thus prevent. With two leylines out, the combo can't be done. Second, Triskellion as an individual card has been useful in the past but it was very hard to ever leverage Mikaeus, the Unhallowed making it feel very bad when it is sitting in your hand. It was possible with Utopia Sprawls naming black and enough untappers but it wasn't practical and wasn't worth additional fetchable green/black sources. Switching to Kiki Conscripts added two more solid playable cards to the deck and kept the splash to red, though I found that the fourth Stomping Ground has been a good idea.
Sideboarding
The sideboard plan is pretty straight forward. Blood Moon comes in against most decks with greedy mana bases game 2, since you have a very good chance of casting it on turn 2 or turn 3. Humans, tron, jeskai and sometimes even mardu pyromancer (which can run Blood Moons themselves) all get badly hurt by going for early fixing/mana efficiency and suddenly becoming a mono red player. It is absolutely worth turning off your Nykthos to stick a Blood Moon since Utopia Sprawl tends to pick up a lot of the slack. Blood Moons often come in for Wisful Selkies due to the triple green being harder to make when you're shutting off some of your green sources and more aggressively getting red sources.
Creeping Corrosion is for affinity and KCI. Dropping one on turn 2/3/4 against either deck can often simply end a game.
Thrun, the Last Troll is mostly for the control matchup. Having a big body that is incredibly hard to deal with can often just beat down a jeskai player or UW player and force them to use their Cryptic Commands to tap your lone Thrun. That always feels pretty good.
Kitchen Finks comes in against burn. It is also a pretty effective card a fighting other aggressive decks like humans and hollow one. Finks are kings of chump blocking and even trade well. They buy time and provide additional devotion.
Reclamation Sages often come in against artifact decks or to beat out enchantment engines like Search for Azcanta
or pop Aether Vials without majorly changing the deck's plan.
Relic of Progenitus beats graveyard decks on its own. Vengevine, Hollow One, storm, pyromancer, GDS all can fold pretty hard to a single relic.
Last up is Dosan of the Falling Leaf. This card is currently in testing but obviously is combo protection. Beat out control cards. The word is still out on how effective this is but blue decks will hate it.
