Whenever a creature enters the battlefield, its controller sacrifices a creature or land.
10/4/2004 If the creature changes controllers before this ability resolves, its new controller has to sacrifice something.
10/4/2004 If more than one of these is on the battlefield, the controller sacrifices a creature for each one of these.
10/4/2004 If the controller has no other lands or creatures on the battlefield, they must sacrifice the creature that just entered the battlefield.
10/4/2004 Tainted Aether’s controller is the one that controls its triggered ability, but the controller of the creature that entered the battlefield chooses what creature they sacrifice during resolution.