Rethink

Counter target spell unless its controller pays {X}, where X is its mana value.

Rhystic Deluge

{U}: Tap target creature unless its controller pays {1}.

Rhystic Scrying

Draw three cards. Then if any player pays {2}, discard three cards.

Rhystic Study

Archival Whorl

Whenever an opponent casts a spell, you may draw a card unless that player pays {1}.

Ribbon Snake

Flying
{2}: This creature loses flying until end of turn. Any player may activate this ability.

Shrouded Serpent

Whenever this creature attacks, defending player may pay {4}. If that player doesn't, this creature can't be blocked this turn.

Spiketail Drake

Flying
Sacrifice this creature: Counter target spell unless its controller pays {3}.

Spiketail Hatchling

Flying (This creature can't be blocked except by creatures with flying or reach.)
Sacrifice this creature: Counter target spell unless its controller pays {1}.

Celestial Convergence

This enchantment enters with seven omen counters on it.
At the beginning of your upkeep, remove an omen counter from this enchantment. If there are no omen counters on this enchantment, the player with the highest life total wins the game. If two or more players are tied for highest life total, the game is a draw.

Stormwatch Eagle

Flying
Sacrifice a land: Return this creature to its owner's hand.

Sunken Field

Enchant land
Enchanted land has "{T}: Counter target spell unless its controller pays {1}."

Troublesome Spirit

Flying
At the beginning of your end step, tap all lands you control.

Windscouter

Flying
When this creature attacks or blocks, return it to its owner's hand at end of combat. (Return it only if it's on the battlefield.)

Withdraw

Return target creature to its owner's hand. Then return another target creature to its owner's hand unless its controller pays {1}.

Agent of Shauku

{1}{B}, Sacrifice a land: Target creature gets +2/+0 until end of turn.

Avatar of Woe

If there are ten or more creature cards total in all graveyards, this spell costs {6} less to cast.
Fear (This creature can't be blocked except by artifact creatures and/or black creatures.)
{T}: Destroy target creature. It can't be regenerated.

Prophecy