Manastorm

Flash
At the beginning of each player's upkeep, that
player flips a coin. If they win the flip, whenever
that player taps a land for mana, that player adds
one mana of any type that land produced until
end of turn. If that player loses the flip,
whenever that player taps a land for mana,
Manastorm deals 1 damage to that player until
end of turn.