Urborg, Tomb of Yawgmoth

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

Toxrill, the Corrosive

At the beginning of each end step, put a slime counter on each creature you don't control.
Creatures you don't control get -1/-1 for each slime counter on them.
Whenever a creature you don't control with a slime counter on it dies, create a 1/1 black Slug creature token.
{U}{B}, Sacrifice a Slug: Draw a card.

Kormus Bell

All Swamps are 1/1 black creatures that are still lands.