Set 1 "Call of the Haunted" from your Deck or GY, and if you do, discard 1 card
during the original animes era, they were considered vital for any deck
Because people like Adam Maysonet were playing all Moxes and Balances and Armageddons, and killing people with Dingus Egg damage
And how many ex cards

Phyresis Outbreak gives poison and might kill opponents creatures Feed the Swarm enchantment removal in black Vat Emergence gets a creature back and proliferates Spread the Sickness destroys a creatures and proliferates Pact of the Serpent strong tribal card draw Living Death swap graveyards with battlefields, great after sacrificing your whole board to draw cards Skullclamp sack small rats for card draw Thrumming Stone if you go the Relentless Rats or Rat Colony route Coat of Arms your rats will be huge Bolas's Citadel this card is dumb lol, lots of casting power and then you hit Gray Merchant of Asphodel and keep going lol Icon of Ancestry kinda draw buffs rats...is ok Contagion Engine proliferate twice each time you activate Eldrazi Monument gives creatures flying indestructible and +1/+1 Grafted Exoskeleton gives a creature infect, great with Crypt Rats Sword of Truth and Justice proliferate sword Bad Moon buff all black creatures, opponents too Phyresis gives infect again good with Crypt Rats Greed card draw Vraska, Betrayal's Sting proliferate planes walker I know that's a lot but hopefully gave you some ideas to work with

Reprinted decks (like the Lord of the Rings ones), and secret lair ones, were left out of the consideration