Rhystic
Rhystic cards were featured in the Prophecy expansion. It's n mechanic on cards that have some effect unless a player pays to keep the effect from happening. For example Withdraw is an instant that reads, "Return target creature to its owner's hand. Then return another target creature to its owner's hand unless its controller pays 1 Mana." When Withdraw resolves, the targeted creature's controller may pay 1 Mana to stop the second part of Withdraw's effect from happening.
As with all spells and abilities, most choices (other than targets) are made on resolution, not when the spell or ability is played. For rhystic spells and abilities, this includes the choice of whether to pay the cost to stop the "unless" part of the effect.
Flavorwise rhystic spells are fast, cheap, reckless spells that can be easily disrupted if there is too much mana around. Invented by Alexi, Zephyr Mage and other Jamuraan spellshapers. Alexi aided the forces of Barrin and Teferi in the Kipamu League against the invading Keldons.
The following cards have the Rhystic mechanic: