Urborg, Tomb of Yawgmoth

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

Sheoldred, Whispering One

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
At the beginning of your upkeep, return target creature card from your graveyard to the battlefield.
At the beginning of each opponent's upkeep, that player sacrifices a creature of their choice.

Blistergrub

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
When this creature dies, each opponent loses 2 life.

Nighthaze

Target creature gains swampwalk until end of turn. (It can't be blocked as long as defending player controls a Swamp.)
Draw a card.