Primeval Protector

Oracle Text

This spell costs 1 less to cast for each creature your opponents control.

When Primeval Protector enters the battlefield, put a +1/+1 counter on each other creature you control.

Card Rulings

11/8/2016 Once a player has announced that they are casting Primeval Protector, no player may take actions to try to change the number of creatures your opponents control in order to increase its cost.
11/8/2016 Primeval Protector’s ability can’t reduce its cost below .