Halloween Sewer Swarm

Will this get you to top 10 masters? Probably not.

Is it a combo you've probably never used or seen used until now? Highly likely.

Seriously, when was the last time you saw Lastlight Druid, Argenport Sewers, Tend the Flock or Calderan Gunsmith played? I mean, sure, it's because those cards kind of suck, but this deck is surprisingly functional! I've even managed some win streaks so it's definitely better than it looks at first glance. Most importantly it's FUN. If nothing else you get to madness+combust/devour a lot.

The idea is to combo Lastlight Druid + Argenport Sewers. If you play Argenport Sewers T4 and then Lastlight Druid T5, it transforms the rats into random 1 drops at the end of T5, summons two new rats at the start of T6, and does the same to them. Repeat for as many nightfall triggers as you have. However this also means everything else you play turns into random trash at the end of your turn. So we mostly use tokens, spells and units with summon effects.

Build Logic
We need to keep the rat population down if we don't have Lastlight Druid to transform them, so we play Devour.

Devour means Madness is really good.

Madness means Combust is really good, so we add fire as a third color. Plus it's another way to keep the rat population down.

Torch is the best card in the game.

With fire and tokens, Rally is really good.

With Lastlight Druid and Rally, Grenadin Drone and Tend the Flock are good.

Lastlight Druid does best with cards that have good summon effects, and Calderan Gunsmith fits well with the tokens + summon effect theme.

Bandit Queen would be really good, but it's really hard to hit FF while also hitting SSS and P, so she's limited to one-of in the market as a finisher.

Each token you summon decreases the cost of The Witching Hour, so it fits in pretty well. Keep in mind that Lastlight Druid will transform them at night, AND THERE HAVE TO BE 4 OPEN SPOTS ON THE BOARD! Only units played from hand give you the option to make room by sacrificing a unit, spells just play units until there's no more available space.

I've gone through several iterations of this so far and this feels pretty close to an optimal list. The power base could probably be tuned a bit, and there may be something better to run than Obrak, but everything else feels good.
Calderan Gunsmith surprisingly does a lot of work as it's easy to have a lot of units on board with this deck. It will regularly get 6+ damage, which is like a cheaper obliterate plus a body.

Example of an ideal game
T1 Grenadin Drone (2 units on board)
T2 Tend the Flock (5 units on board)
T3 Kerendon Merchant (6 units on board)
T4 Sewers (6 units on board)
T5 Lastlight Druid + Grenadin Drone (11 units on board)
T6 Calderan Gunsmith for 12 to the face
T7 Rally/Bandit Queen, a-space

Noteworthy interactions:

Sewers will summon rats before Obrak the Feaster's sacrifice effect triggers, so he'll be able to sac a rat instead of himself. :)

Sewers checks for the number of rats on the board BEFORE Lastlight Druid's transform effect hits, so even if your 3+ rats are transformed, Sewers will still destroy itself :(

Even though Calderan Gunsmith does each damage individually like a machine gun, it still counts as one effect and so all damage will be blocked by face Aegis.

EDIT: Obrak's been underperforming so I'm testing out running Assembly Line instead. I've also added pretty much every card that could be considered viable in this deck as sideboard cards. If you're looking to try out this deck but are missing cards, or want to improve it, I'd start there.