Relentless Rats

Relentless Rats gets +1/+1 for each other creature on the battlefield named Relentless Rats.
A deck can have any number of cards named Relentless Rats.

Mogis's Marauder

When Mogis's Marauder enters, up to X target creatures each gain intimidate and haste until end of turn, where X is your devotion to black. (A creature with intimidate can't be blocked except by artifact creatures and/or creatures that share a color with it. Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Elves of Deep Shadow

{T}: Add {B}. Elves of Deep Shadow deals 1 damage to you.

Forest

Swamp

Life's Finale

Destroy all creatures, then search target opponent's library for up to three creature cards and put them into their graveyard. Then that player shuffles.

Gray Merchant of Asphodel

When Gray Merchant of Asphodel enters, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Doubling Chant

For each creature you control, you may search your library for a creature card with the same name as that creature. Put those cards onto the battlefield, then shuffle.

Bloodbond March

Whenever a player casts a creature spell, each player returns all cards with the same name as that spell from their graveyard to the battlefield.