This deck attempts to use toxic as its wincon with nearly all of the spells and creatures focussing on toxic, posion counters, or proliferate. Toxic's power can swing massively, especially in pauper where the amount of cards with decent toxic values is incredibly limited. Abzan (W/G/B) provides some of the better options within pauper for toxic decks, but it can still be very hit or miss.

Main Deck:

The creatures in the main deck revolve around expendable 1/1s with a handful of larger utility creatures thrown in for either support or to deal a fatal blow. The idea is to overrun your opponent before they can mount a decent defence using mites, profilerate abilities and the flying Pestilent Syphoner to inflict as many poison counters in the early game as possible. Plague Nurse is a key ingredient. It provides a buff to the smaller toxic creatures, allowing simple 1/1s to deal 2 or 3 toxic damage each, but also has a decent body itself to block attackers and provide a line of defense whilst your smaller creatures go in for the attack. Crashing Drawbridge gives all of your creatures haste, making those small attackers all that more lethal in the early game.

The spells in the main deck are straight forward - they either profilerate, give poison counters, or give card draw. Cards like Wedding Invitation give you extra cards in hand but also pair nicely with Plague Nurse to create big, unblockable creatures to dish out a massive amount of toxic counters in one fell swoop.

The mana base is also fairly straight foward. The depletion lands work with the proliferate mechanic to give you plenty of mana, especially for the Plague Nurse's ability, with basic lands thrown in so they can be tutored up by Thirsting Roots if needed.

Sideboard:

With no creatures in the sideboard, all the spells available are to fill gaps in the main deck depending on what your opponent is playing or if the deck begins to suffer in one particular area. Theive's Tools are there to replace Wedding Invitation, largely due to the unblockable ability of the Theive's Tools. Theive's Tools will lose you the card draw, but you will gain a bit of extra ramp in its place. Blood Fountain also provides card draw with its blood token, but is a good target for Carnivorous Canopy, giving two low mana artifacts that can be sacked to proliferate.

Vraska's Fall and Whisper of the Dross provide a bit more removal with the added bonus of either proliferate or a direct poison counter to kick your opponent's completion off. Tormod's Crypt gives a small amount of graveyard hate, while Double Cleave allows for your unblockable creatures or the Tyrranax Atrocity to deal massive amounts of poison counters in one go.

Alternatives:

Blightbelly Rat could be swapped in for Branchblight Stalker. It would provide an extra instance of proliferate, but it's ability to inflict poison counters on the battlefield is more limited that Branchblight's. Deadly Dispute would be another good card to swap in for either Infectious Inquiry or Carnivorous Canopy. If Blightbelly Rat is added to the deck, Deadly Dispute might make more sense than Infectious Inquiry as you could get 2 cards, a treasure token and a proliferate off of only 2 cards. Sheoldred's Headcleaver could be swapped in for either Tyrranax Atrocity or Basilica Shepherd for another unblockable toxic creature, but would lack the upside of being speedy like Tyrranax or provide mites like the Shepherd.