Sheoldred, Whispering One {5}{B}{B}

Legendary Creature — Phyrexian Praetor

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.

6/6

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If another triggered ability during your upkeep causes you to sacrifice a creature, such as that of Lord of the Pit, you won't be able to target it with Sheoldred's triggered ability as that ability is put on the stack. (2017-11-17)
The opponent whose upkeep it is chooses a creature to sacrifice when Sheoldred's second triggered ability resolves. (2017-11-17)

In a Two-Headed Giant game, the last ability will trigger twice at the beginning of the opposing team's upkeep, once for each player on that team. Each player sacrifices only a creature they control. (2017-11-17)
Swampwalk is a keyword ability that means “This creature can't be blocked if defending player controls a Swamp.” Most nonbasic lands don't have basic land types, even if they produce colored mana. For example, Graven Cairns is neither a Swamp nor a Mountain, while Blood Crypt is both. (2017-11-17)