Whenever one or more modified creatures you control attack, untap all modified creatures you control. After this combat phase, there is an additional combat phase. This ability triggers only once each turn.
Whenever Akki Underminer deals combat damage to a player, that player sacrifices a permanent.
"Deep inside the Sokenzan Mountains, a band of akki discovered a cache of ancient items of power. Their ensuing spree of destruction became known as 'The Three Days of Fun.'" —Observations of the Kami War