Rat Plague

>Gameplan

We have a quite aggressive gameplan: We want to get as many rats as possible onto the battlefield to grow our Pack Rat and Rat Colony gigantic over the course of the game, until they (and the sheer mass of rats we have) can close the game out.

Disruption

As we first need to grow our rats before they are able to just eat every other creature on board, we have Doom Blade and Go for the Throat to deal with threats our rats are not able to deal with in time.

The Swarm

Our game-ending rats are Rat Colony and Pack Rat, for a total of 14 threats. Pack Rat also can copy itself at the cost of discarding cards. For additional tribal payoff we have a full playset of Piper of the Swarm, being able to pump out more rats without having to spend cards and being able to steal the opponent's threats, possibly taking their win-condition away and ending the game by ourselves. This opens up the possibility of turning games around pretty fast. The most important thing about Piper of the Swarm is that it provides our rats evasion with menace. Having to chump-block with two creatures each time in the late game to prevent you from dying is not something the most decks can manage to do.

Fuel

As discarding cards for Pack Rat to create more big tokens is a thing that we want to do and to keep our hand filled with cards, we play Sign in Blood.

Spam the Board

To bring the rat count up quickly, we play literally every rat one drop available in Modern (except changelings). We don't really care about their abilities, the quantity is important. Typhoid Rats can be nice because of deathtouch hindering the opponent from attacking us with their threats.

Manabase

We play 22 lands, four of them being Castle Locthwain, enabling even more card-draw.

>Sideboard

In the sideboard we have Lurrus of the Dream-Den, which is NOT a part of the actual strategy. It was just a free companion and had a bit of synergy with Pack Rat, as it can resurrect our discarded permanents. You can easily exclude it from the list and not much will change.
This deck rises and falls with the opponent either playing mass removal or not. If the opponent doesn't, we have a good chance of winning, as he is just not going to be able to deal with all the rats, with some of them being menacing eldrazi-like gigantic 13/1s, attacking them and beating them down eventually.
If board wipes (or Plague Engineer) are a thing in the opponent's deck, we will probably have a lot of trouble killing the opponent, as we really rely on the quantity of creatures on our board.
The deck is able to win more grindy match-ups, as it has quite a lot card draw and even resurrection with Lurrus of the Dream-Den.

Tron:

Against tron, we play Ghost Quarter, which we will swap out for Castle Locthwain, when sideboarding.

Dredge:

When playing against dredge, we will just have to hope on overwhelming the opponent with our stuff, rather than destroying his stuff. So we will swap out our entire removal arsenal for Tormod’s Crypt. I don't want to run Grafdigger's Cage, as it shuts Lurrus of the Dream-Den off.

Burn:

Sadly, we don’t have a whole lot to do against burn. The only real thing we can do, is to aggro it out.

Mill:

Against mill, we don’t even need to do something against that deck. We will just remove their crabs with our removal, and go for beatdown.

>Budget

This list is something I would call very budget, as it's a functioning 40€ deck in the modern format.