Ra'thal Sanctuary

During any opponent's turn, if you control a
non-black creature with power 3 or greater that
did not attack during your last turn, you may tap
any number of untapped lands you control for
mana and "store" that mana in Ra'thal Sanctuary.
You may use this stored mana only during your
next turn.
At the end of your turn, remove all stored mana
from Ra'thal Sanctuary.