Mistcaller

Creature — Merfolk Wizard

Sacrifice Mistcaller: Until end of turn, if a nontoken creature would enter the battlefield and it wasn't cast, exile it instead.

1/1

2018-07-13:
Mistcaller’s ability won’t affect any nontoken creatures that were cast, including ones cast from unusual zones such as your graveyard.
2018-07-13:
Mistcaller’s ability doesn’t stop creature tokens from entering the battlefield. It also doesn’t affect creatures that are already on the battlefield.
2018-07-13:
If Mistcaller enters the battlefield at the same time as other creatures, its ability can’t be activated in time to affect those creatures.