Strength. Brutality. Monstrosity.
Underworld Cerberus | Art by Svetlin Velinov
Polukranos, World Eater is what we lovingly call a fatty. It's big for its mana cost and comes backed with an ability to clear away pesky creatures that might block its path. What sets Polukranos apart is its color: green. Typically, green decks have a hard time outright destroying others' creatures, but Polukranos neatly solves this problem if you have a bucket of mana to dump into it.
There are obvious ways to take advantage of Polukranos becoming monstrous. Basilisk Collar and Sword of Kaldra might be the most potent possibilities, but I'd settle for the even more flavorful Gorgon Flail to turn a one-time monstrous rampage into a slaughter of creatures. And with Polukranos as your commander, the odds are good you'll be able to wash, rinse, and repeat a time or two more to if needed.
What Polukranos lacks in build-around-me possibilities compared to Anax and Cymede, it more than makes up for in the power it brings on its own to the battlefield. What else would we expect of a World Eater?
That's something you've helped to answer.
You requested some Commander lists based around the new Theros legends, and since there is even a Hydra-subtheme I figured a "My spell is always bigger than yours!"-Hydra themed deck with Polukranos would work out just fine.
The deck wants lands and more lands to synergize with all the "Heartbeat of Spring"-ish cards I could think of. I figure that if my deck always plays a creature that's bigger than anything my opponent can produce with a commander that can clear the entire board at will, I should have the upper hand even if I feed the entire table all the mana in the world. The rest of the deck has its focus on cards that give extra value from counters and huge dudes, like Greater Good, Feed the Pack, Manaplasm or any of the cards with a fight mechanic (and a Stuffy Doll was too tasty not to add with all that fighting). I also added a few tutors to find bullets when I need them and to re-use my graveyard, since I figure it might be nice to get all those Hydras back.
Funky tribal decks are always a treat, and the explosion of Hydra options in recent Magic sets ensures there's plenty of friends for Polukranos to party with. Going big and becoming the biggest is always a fine plan in Commander, and any deck that gives everyone the mana to make their dreams happen is a deck I like to play with!
Polukranos's monstrous ability is where all the ideas in this deck are derived from.
There are many cards focusing on the +1/+1 counters themselves—the graft and evolve creatures, especially. Gyre Sage goes insane with mana, Renegade Krasis builds up my entire team to an astonishing level, Fungal Behemoth distributes counters like nobody's business, etc.
Fighting is another theme, and Darksteel Plate, Rite of Passage, Asceticism, and Voyager Staff help my creatures survive. Lifelink and deathtouch are synergistic with fighting too, with Vial of Poison or Simic Basilisk turning Polukranos into a green Plague Wind.
The ramp not only makes Polukranos's monstrous ability excel, but there are lots of other high-impact cards here like my Hydras and other X-things that benefit from it. Woodfall Primus and the persist granted by Cauldron of Souls is a clever trick to take advantage of all the +1/+1-counter love to make my guys hard to kill.
I'll admit it: this deck won me over by featuring Fungal Behemoth as a card. There's just something I love about finding ways to make +1/+1 counters the center of a Commander deck. Whether it's to keep on ramping with Fertilid or reset the persist of Woodfall Primus, playing with +1/+1 counters ensures your opponents will have to watch out for more than just the namesake monster at the deck's head.
Polukranos offers an interesting challenge, with no obvious hints on how to build a deck around him. Looking deeper, the World Eater is an amazing commander who can shore up mono-green's primary weakness (dealing with creatures).
Polukranos wants two things to take over the game: to gain deathtouch so he can kill multiple creatures regardless of size, and the ability to activate his "monstrous" power more often. This deck runs several unexpected Commander cards, such as Wasteland Viper and Nightshade Peddler, to achieve the first goal, and runs a combination of self-bounce cards and sacrifice outlets to achieve the second. The deck supports its strategy with plenty of additional deathtouch creatures and "fight" effects to act as removal, as well as an "enters the battlefield" ability subtheme that can help grind out advantage over a long game.
As it stands, the deck is mid-level in terms of budget and power. One can feel free to increase the power and budget with big-ticket items like Gaea's Cradle or Sword of Light and Shadow, or keep things lighter by adding more creatures with "enters the battlefield" abilities and cheap recursion spells in place of the expensive or "mean" cards currently in the deck. Good hunting!
It's easy for anyone to get caught up in finding "the strongest" cards to include in a Commander deck and miss out on easy ways to get what you really want: synergy. Nobody would expect Wasteland Viper in Commander, but it works great here since it's both a combat trick after blockers are declares and itmixes well with the fact you can make Polukranos (or any other creature with monstrosity) monstrous whenever you want.
To get more than one monstrous activation from Polukranos it will have to leave play then come back, but getting to wipe away whole armies is worth the mana to make it happen.
The paths of our destiny do not end with Polukranos, World Eater. There are two others to follow.