Rats, rats, we're the rats. We prey at night, we stalk at night - we're the rats!

The deck aims to discard your opponent's cards while summoning obscene amounts of rats. The non-rats are your QUEENS except Ogre Slumlord; these queens will need some henchmen to support their rats.

Budgeting

For those on a tighter budget, you can remove the copies of Plague of Vermin from your sideboard and replace a copy of Marrow-Gnawer with something else; Beseech the Queen can still find your remaining Marrow-Gnawer. Diabolic Tutor is a little cheaper than Beseech the Queen and it searches without restriction. Lastly, Geth's Grimoire can be replaced by some draw spells or Dark Withering from the sideboard.

Of course, I am pretty inexperienced myself, so your choices will probably trump mine.

Cards

Creatures

Drainpipe Vermin, Burglar Rat, and Rotting Rats are cheap rats with good discard effects.

Pack Rat and Piper of the Swarm provide a way to produce rats when floating mana; Pack Rat makes chonker rats while Piper of the Swarm can even steal creatures at the cost of three rats.

Ayara, First of Locthwain is very crucial for staying alive, drawing cards, and even ending the game by pumping damage out from Marrow-Gnawer or Plague of Vermin.

Ogre Slumlord dissuades your opponents from attacking into deathtouch rats.

Piper of the Swarm and Marrow-Gnawer provide menace and fear respectively, which can be important for getting the last few points of damage in to win the game.

Non-creature spells

Geth's Grimoire can provide copious amounts of card draw especially against many opponents (see Burglar Rat and Rotting Rats).

Unearth can return Ayara, First of Locthwain or Liliana, Heretical Healer to play. The rats are also fine options.

Bad Moon is a cheap way to keep your rats strong and healthy.

Lands

As a mono-black deck, swamps make up the majority of your mana. Castle Locthwain is included for some added card draw and flavor for Ayara, First of Locthwain.

Questionable Cards

Liliana, Heretical Healer does not need to be here, but I like the thought of attaining never-ending swarms of rats from Liliana, Defiant Necromancer.

Tergrid, God of Fright is also questionable, but she has strong potential with the many discarding effects.

Beseech the Queen to draw the exact legendary creature you want. Dark Tutor is probably a better choice, but this is a budget deck.

Sideboard

Chittering Witch is very weak in a one-opponent game, but with three or more opponents, she is pretty sexy.

Dark Withering provides the only consistent removal. It can be discarded and played for its madness cost through the effects of Drainpipe Vermin, Pack Rat, Rotting Rats, and Liliana, Defiant Necromancer.

Nezumi Bone-Reader can mass-sacrifice rats to potentially discard your opponent's entire hand.

Mimic Vat is an interesting way to continuously return your Marrow-Gnawer to play just to use his tap ability. Other good targets include Chittering Witch; Tergrid, God of Fright; and even Pack Rat.

Plague of Vermin is flavorful and game-winning. With Ayara, First of Locthwain in play, the cost of life is negated and obliterates opponents if you have more life than them.

Bojuka Bog for the graveyard hate.