Power Siphon

Whenever an opponent taps a land, tap another
target land they control, then add one mana of
any color to your mana pool. That mana
doesn't empty from your mana pool until the beginning of your next end step.
"Why shouldn't I steal their mana when it's so vulnerable? What's the point of a weakness if it's not
being exploited?"
-Schkath, Mana Plagued