If the creature changes controllers before this ability resolves, its new controller has to sacrifice something.

If more than one of these is on the battlefield, the controller sacrifices a creature for each one of these.

If the controller has no other lands or creatures on the battlefield, they must sacrifice the creature that just entered.

Tainted Aether's controller is the one that controls its triggered ability, but the controller of the creature that entered chooses what creature they sacrifice during resolution.