This deck is a moderate budget upgrade to the Atraxa, Praetors' Voice precon Breed Lethality. The build is for a moderate power level, it is above casual level but it is definitely not competitive. The deck wins by beatdown usually, as it is not hard to get incredibly large creatures that swing for 15+. It can however combo out and kill all of your opponents as an alternate win-con.
I chose not to include tutors to keep its power level down. If you want to upgrade the deck, they are very easy cuts you could make to include tutors to increase the reliability of the combo.
That being said, the combo is as follows:
You need Rite of Passage down on the field, as well as Corpsejack Menace/Winding Constrictor/Hardened Scales/Pir, Imaginative Rascal (any of those four will work). Once you have those two pieces on the field, you need to cast Walking Ballista for at least 4 mana, allowing it to enter with 2 +1/+1 counters, which Corpsejack Menace/Winding Constrictor/Hardened Scales/Pir, Imaginative Rascal turn into 3 or 4. Then, you remove a counter from Walking Ballista and have it deal 1 damage to itself. This triggers Rite of Passage which would put a counter on Walking Ballista. Then Corpsejack Menace/Winding Constrictor/Hardened Scales/Pir, Imaginative Rascal triggers, causing Rite of Passage to place two counters instead.
Everytime you repeat this process, Walking Ballista nets one +1/+1 counter. You repeat this until you have an arbitrarily large amount of counters on Walking Ballista, then you ping all of your opponents down for their life totals. (Note: this needs to be done on the next upkeep so the damage is removed from Walking Ballista, allowing us to remove as many counters as we need).
The combo is not incredibly reliable, since it folds to a Krosan Grip, and we don't have tutors to fetch our pieces. Most of the time that I win with this deck, it is either through having a huge creature or by Atraxa, Praetors' Voice commander damage. The beatdown portion of this deck is fairly straight forward, and doesn't warrant much of description in my opinion.
Another combo in the deck is one for infinite life, which is done by having Archangel of Thune and Spike Feeder on the battlefield. This allows us to remove a counter from Spike Feeder and gain 2 life, which triggers Archangel of Thune to put a +1/+1 counters on Spike Feeder and every other creature we control. We can repeat this process again and again, gaining an infinitely large amount of life and all of our creatures other than Spike Feeder get infinitely large as we continue to add +1/+1 counters to them.
Some planeswalker spells are included because of Atraxa, Praetors' Voice's ability to proliferate their loyalty counters to provide them with extra survivability and value. So if you can replace a card with a planeswalker that can do the same job (like Ob Nixilis Reignited in the place of Underworld Connections), it is worth the consideration simply due to our commander.