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

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.

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.

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.