Etched Floodgate of Tideguard

At the beginning of your upkeep, if an
opponent controls five or more creatures,
sacrifice Etched Floodgate of Tideguard.
When you do, return all non-land
permanents to their owner's hands.
Engineered by the master metalworkers of the
south seas centuries ago, it periodically floods the
land, purging it of impurities.