Tetnakal, Furious Respiral

This card costs {x} less, where X is equal to the total power and toughness of all creatures.
Trample, Double Strike, Menace, Haste, Indestructible, Flying, Mountainwalk, Forestwalk, Swampwalk, Islandwalk, Plainswalk
When Tetnakal, Furious Respiral attacks, destroy all nonland permanents defending player controls.