Pygmy Hippo

Whenever Pygmy Hippo attacks and isn't blocked, you may have defending player activate a mana ability of each land he/she controls and empty his/her mana pool. If you do, Pygmy Hippo assigns no combat damage this turn and at the beginning of your postcombat main phase, you add [Variable Colorless] to your mana pool, where X is the amount of mana emptied from defending player's mana pool this way.