Ransom

Ransom are two different mechanics (rule-wise) that have the same name (and they are very similar flavour-wise, and thus could be considered as one) that appeared on two test cards in the Mystery Booster set. One is a keyword ability, that allows a player to undo the effect of a card by paying a ransom cost. The other one is a keyword action, which allows to exile a card, and lay it exiled, until the owner pays to return it to their hand.

Description
Unlike similar cards that allows the opponent to pay a resource to keep the effect from working, like Force Spike, the Ransom cost can be paid at any moment if having priority.

Rulings

 * Paying a ransom cost to end an effect is a special action. That cost may be paid any time if having priority, and players can't take actions in between the time the player announces their intent to pay and the time the special action resolves.
 * In the keyword ability case, if a card with a ransom ability loses that ability, it has no ransom cost. No player may pay its ransom cost.
 * In the keyword action case, the exiled card's owner may pay its ransom cost even if the permanent that ransomed it leaves the battlefield.