Emma put some in the onion rings she gave Zelena, to accelerate her pregnancy; she wanted baby Robin out of the way, so she could put the Darkness into Zelena and then kill her with Excalibur.
Emma put some in the onion rings. She gave Zelena. This was to accelerate her pregnancy some. She wanted baby Robin out of the way. She could put the Darkness into Zelena. She could kill her with Excalibur.