Sheoldred, Whispering One [The List] Elder Dragon The first instant or sorcery

The first instant or sorcery spell you cast each turn costs U (or 1) less to cast for each land you control with a flood counter on it

that source's controller sacrifices that many permanents

Whenever the Llanowar Elves gather the fruits of their forest

believe that humans sprang forth from Merfolk who adapted themselves in order to explore their last frontier

Kathril chooses when it is seen

Sheoldred, Whispering One [The List] Elder Dragon The first instant or sorcerySet: The List Type: Legendary Creature Phyrexian Praetor Rarity: Mythic Cost: {5}{B}{B} 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.