Sheoldred, Whispering One

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.

The opponent whose upkeep it is chooses a creature to sacrifice when Sheoldred's second triggered ability resolves.

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.

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.