The opponent whose upkeep it is chooses a creature to sacrifice when Sheoldred's second triggered ability resolves.
Sheoldred, Whispering One
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.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Jana Schirmer & Johannes Voss
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