Put X +1/+1 counters on target creature you control. Then counter up to one target spell unless its controller pays mana equal to the greatest power among creatures you control.
"Remind me to send the biomancer team a gift basket."
—Olana, Simic field researcher
If you control no creatures as Repulsive Mutation is resolving, the amount of mana the target spell’s controller must pay to stop their spell from being countered is 0. That player can choose not to pay 0 mana; if they do, the spell will be countered.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.