Jet Medallion

Black spells you cast cost {1} less to cast.

Marrow-Gnawer

All Rats have fear.
{T}, Sacrifice a Rat: Create X 1/1 black Rat creature tokens, where X is the number of Rats you control.

{2}, {T}: Add {B} for each Swamp you control.

This creature gets +1/+0 for each other Rat you control.
A deck can have any number of cards named Rat Colony.

Each land is a Swamp in addition to its other land types.

Black creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, each opponent loses 1 life and you gain 1 life.