Oh look I can shove the Revilark/Karmic Guide combo in a Roon Deck. Booooo.
We can have so much more fun than that.
This deck started out as Jenara, Asura of War deck but she was sort of a dead card. There was always better stuff to play.
So when Roon of the Hidden Realm was released we evolved.
There are two core synergies in this deck, the first is with Roon. There are a lot of enters the battlefield triggers, Avenger of Zendikar, Cloudgoat Ranger and the other 5 mana token generators, along with general value creatures like Eternal Witness, Angel of Sanctions, Farhaven Elf, Knight of Autumn (replacing Harmonic Sliver), and Prime Speaker Zegana. Then of course the persist creatures can give us even more value (more on them later). But there are even deeper interactions. Devour is an enters the battlefield mechanic so Roon gets a chance to re-trigger all those mechanics, on cards like Skullmulcher you can devour over and over, Mycoloth can devour his own tokens to get bigger and so on. You can retrigger the Polukranos, World Eater mythic ability to repeatedly wipe the board. Then there are the "enters the battlefield" triggers on other cards you won't themselves cycle out - Bloodspore Thrinax, Juniper Order Ranger, Master Biomancer, and Cathars' Crusade. Every time you blink a creature out it will retrigger these bad boys, and as should be obvious with your persist creatures this gets even more insane. Any persist creature and one of these plus Greater Good is functionally infinite cards and with Woodfall Primus also blows up their board. With just Glen Elendra Archmage and a +1/+1 counter maker you can have a very close to hard lock that's dependent really on just your blue mana. Then of course Bramble Sovereign and Progenitor Mimic are other power players except for copies as opposed to +1/+1 tokens.
The second core synergy is with Doubling Season, and to an only slightly lesser extent Primal Vigor. The obvious is that nearly everything here uses counters or tokens - and oftentimes both (Mycoloth is a prime example - devour 2 creatures, get 8 +1/+1 counters making 16 saprolings). Obviously you can make Rite of Replication and Martial Coup huge but there are other interactions worth noting. Little things like Coalition Relic being able to make you more mana, or if you want to run the storage lands like Saltcrusted Steppe you get double the storage counters (I generally don't as you usually want to be tapping out most turns), your planeswalkers can all become crazy immediately with Doubling Season. You can play Elspeth, Knight-Errant to instantly get an indestructible emblem for instance. Umezawa's Jitte can become removal even for fatties. But maybe my favorite interaction is if you can get out Doubling Season, Contagion Engine, and Parallax Wave (or even without Doubling Season). With Doubling Season and Contagion Engine out, even forgetting about its own -1/-1 counters (which sadly don't double via Doubling Season) you can then play Parallax Wave with 10 fading counters. Then you are able to, through Contagion Engine + Doubling Season, add 4 more fading counters per turn - even without doubling season that's 2 per turn. This essentially means a permanent wipe on all or nearly all creatures you want on the enemy board. You want to be a real asshole you can find a way to shove Parallax Tide into the board for even more power - however I don't find it as useful alone compared to Parallax Wave.
To search out these enchantments along with your other powerful enchantments is a suite of enchantment search, Academy Rector, Enlightened Tutor, Idyllic Tutor, and Sterling Grove.
The main reason to play this doubling season deck over G/W/B or G/W/R doubling season decks is the card advantage. Greater Good, Skullclamp, the staple Rhystic Study, along with Fathom Mage, Prime Speaker Zegana, and Skullmulcher (Skullmulcher is the weakest of these, its worth considering replacing with Consecrated Sphinx, its less synergy but more raw power) you can create crazy amounts of card advantage while generating an unstoppable swarm of fatties.
There are other little synergies worth noting. Gyre Sage can make a lot of mana and Fertilid can search up all your basics very easily. Rishkar, Peema Renegade can turn all your creatures into llanowar elves without breaking a sweat and synergizes with both Gyre Sage and Fertilid as well as Fathom Mage and others. Also, make sure to devour your own Academy Rector.
Minamo, School at Water's Edge can untap both Roon but also Gaea's Cradle.
It's worth noting that I don't include Parallel Lives or Anointed Procession as they only have half synergy - I'd rather find room for card advantage or tutoring effects.
This deck can ramp up to a pretty unstoppable boardstate that is able to dominate the redzone and turn 0/1 plant tokens into creatures that are staring down enemy finishers like they're nothing. Even without Doubling Season there is crazy synergy going on here. But it's how all these synergies come together that really makes it fun. Imagine for instance I have Roon of the Hidden Realm, Doubling Season, Bramble Sovereign and Geist-Honored Monk. When you remove the monk and return it, you can make a copy, with doubling season 2 copies, then each of the two copies and the original each make 4 creature tokens - 12 tokens. Now, just for the hell of it, activate Gavony Township (or one of a dozen other ways to put +1/+1 counter on each creature you control). Bonkers. Now imagine you have Prime Speaker Zegana in hand. It's exponential. Everything builds on everything else.
However it does have weaknesses. Mass and spot removal can both be a problem. If you want to be more competitive maybe find a way to fit Asceticism into the deck. Twilight Shepherd helps certainly, and usually you have enough value creatures to come back after the first boardwipe - but two in succession, especially ones that hit more than just creatures (Akroma's Vengence, Nevinyarll's Disk) can be really tough. Play conservative with Roon, if you have a key creature wait to use his ability to save that creature so you can bounce back faster.
It's also prone to get combo'd on, especially spell heavy combos, as this deck does not have a lot of permission or other disrupting elements. It does a little better against creature heavy combos as it has a decent amount of removal of its own, but it can still get victimized. Again if this is big in your playgroup find a way to fit in Ethersworn Canonist or some other sort of combo-stopping elements that won't also hinder you too bad.
However if someone wants to fight in the redzone this deck can outgun anything out there. Everything builds off itself, and it you are allowed to untap with Doubling Season for even a turn, God forbid two, you'll go nuts.
Come prepared with a lot of dice and token cards. You're gonna need it.
Everything in the maybe-board are very valid cards to run in this deck, and didn't make the cut by very very narrow margins. If you think they work better for you then other stuff in the deck, or have some of the ones there but not the ones I have shown in the main - that's totally valid. And if you want to shove Reveillark and Karmic Guide in there it would probably still be good.
