Why is this Deck?
Because Insidious Roots is awesome. Well, it's not traditionally awesome in the way Golgari graveyard strategies often work, but it happens to synergize with a lot of cards that also don't get the love they deserve.
A ton of cards have the downside of exiling themselves out of the graveyard so you can't recur them, but because Insidious Roots wants creature cards to be leaving the graveyard, we can take this downside and turn it into a massive benefit.
The Graveyard Exilers
Undead Butler is a card I've thoroughly enjoyed in reanimation strategies because it mills but also nets you a raise dead if it's useful for you late game. Alongside Insidious Roots, it not only fills your graveyard but also exiles itself from your graveyard and then, afterwards, brings a creature card back to your hand. Those are two separate instances of creatures leaving the graveyard and, therefore, two Insidious Roots triggers. Skyfisher Spider is also unrestricted removal on ETB (which the deck otherwise lacks) and it exiles itself out of the yard if it dies, getting another trigger.
Tyvar, Jubilant Brawler has appeared in a few janky brews or Commander/Brawl decks, but I have yet to see a competitive deck that leaned into what he brings to the table. He has two massive benefits in this deck because his negative two ability can reanimate something (triggering Roots) but his passive also allows the plant tokens to tap for mana the turn they enter. Given how cheap our other creatures are, this can result in going mana neutral on a number of effects.
The Mill
A number of the cards we've already discussed can mill to fill the graveyard, but Gnawing Vermin has the added benefit of trading with two-toughness creatures to slow down aggro. Also, we have a powerful Surveil package because it's milling except, we make sure we don't mill the Insidious Roots or lands we need.
Snarling Gorehound consistently punches above its weight class by offering a Surveil every time another small creature enters, including our plant tokens because they enter with zero power. Playing a Snarling Gorehound on turn one into a Rubblebelt Maverick gives you a chance to effectively Surveil three if you mill away the first one you see. When Surveilling, don't be afraid to mill aggressively. If the top card isn't a non-creature setup piece or a necessary land, pitch it. The graveyard needs a critical mass of creatures, and moving cards out of the way gets you closer to the engine cards you need.
MTG ARENA NOTE - When stacking mill and surveil triggers on Arena, be sure to manually order them. There is an option in the gameplay menu labeled “Auto Order Triggered Abilities” that needs to be checked off. As of this writing, the client prioritizes Surveil to resolve before mill, so the top card will get milled even if you chose to keep it there.
The Cauldron Combo
If you've looked at the list above, you probably noticed Agatha's Soul Cauldron. Its primary purpose here is to exile creatures out of our yard at instant speed. If we've successfully milled, this is a clean way to take creatures like Gnawing Vermin that can't exile themselves and turn them into Insidious Roots triggers.
I did include one combo that could take advantage of the cauldron if the opportunity presented itself with Soulcoil Viper. If this card ends up under the cauldron, all our plants can sacrifice to reanimate creatures because they all natively have counters on them. Additionally, if Tyvar is online, they can do so instantly. It creates a loop where the newest plant sacrifices itself to reanimate a creature, Insidious Roots triggers, then the new plant can sacrifice to bring back another creature. If the creatures that are coming back mill, this can go on for a long time.
Okay, silly loops are great, but does that win? Yes, because the other plants get an additional +1/+1 counter for every Insidious Roots trigger. Often, you'll start the turn with a 2/3 plant and a 1/2 plant only for each of them to be over ten power by the time combat rolls around. Given how little mana is needed to pull this off, it's more competitive than it looks.
The Glue That Holds it Together
At the end of the day, if this deck does the thing, we put a lot of power on board and win through combat damage. In order to get there, we need to chump early and prioritize setting up our engine. A card that single-handedly does this for us is Mosswood Dreadknight.
This Golgari bomb gives us reasonable stats on board that can tangle with early aggression or put more controlling opponents on a clock they need to answer. Pressure and survivability are both clutch, but its recursion ability also triggers Insidious Roots, so it's super useful both early and late. The only notable downside to the card is its three power makes it too big to trigger Snarling Gorehound, but I'm not about to complain that my two-drop is too powerful. That's silly.
Notable Exclusions
There are a number of brewers trying to break this card right now and many are seeing success with shells that don't match mine too closely. This is awesome and it really speaks to how powerful Insidious Roots is.
There are Sultai variants running Grolnok, the Omnivore, and Willow Geist. Both of these cards are near and dear to my heart because of brews I made years ago now, but I don't believe they're that important in this build. Let's address each of them in turn.
Grolnok, the Omnivore says that any milled permanent immediately goes to exile with a croak counter on it and Grolnok's passive lets you play them. The deck is nearly all permanents, so most milled cards exile, and each creature exiled this way would trigger Insidious Roots. The theory is sound, and the card is strong, but it's a 3/3 for four and if it dies, you lose access to the exiled cards with croak counters. While each card is exiled individually and, therefore, triggers Roots individually, I still don't believe this effect is consistent enough to merit adding another color. It feels “win-more.”
Willow Geist is an amazing card that I have a soft spot for because it's a 1/1 with trample. That's my favorite stat line and nothing can change that. Additionally, it grows as cards leave the graveyard, so it can get absolutely huge. Prior to Insidious Roots being printed, I brewed around this ability to pretty good success and found it very valuable. The problem is, Willow Geist doesn't help us mill or exile the cards and it is, effectively, just an upgraded plant token that requires us to spend a full card. I believe prioritizing creatures that help us get the engine going and keep it running are a better use of card slots.
The Lone Sorcery
This deck needs a ton of creatures in order to be successful and, as a result, we don't have many non-creature spells in it. The exceptions are all key engine pieces or Diabolic Intent. Intent is powerful for two reasons: 1) it helps us tutor up the Insidious Roots to make sure we can actually do the thing, and 2) it gives us a sacrifice outlet if a Roots is already online to help fill our graveyard or get Undead Butler/Skyfisher Spider triggers. Efficient tutor effects can be very powerful in combo strategies, and this is no exception.
You'll almost always search for Insidious Roots because they're critical and better in multiples, but Tyvar, Cauldron, or the spider are also very impactful depending on the board state. Heck, even just replacing a Snarling Gorehound or Mosswood Dreadknight might be the boost you need to close a game out.
The Bottom Line
Insidious Roots is a very powerful card that gives you a heck of a lot for two mana, but it asks you to do a very specific activity that you weren't going to do already. As a result, it's my favorite kind of build around. Rewarding players for drawing cards or hitting land drops can be fun and all, but being challenged to have creature cards leave your graveyard regularly is an interesting mechanical gimmick that was very fun to brew. Additionally, where this deck landed feels very strong and I'm going to be playing it for a long time to come.