Manabarbs

Whenever a player taps a land for mana, this enchantment deals 1 damage to that player.

Mana Flare

Whenever a player taps a land for mana, that player adds one mana of any type that land produced.

Xantcha, Sleeper Agent

Xantcha enters under the control of an opponent of your choice.
Xantcha attacks each combat if able and can't attack its owner or planeswalkers its owner controls.
{3}: Xantcha's controller loses 2 life and you draw a card. Any player may activate this ability.