Astéri, World Ender

Costs {x} less to cast, {x} = # of dinosaurs in play.
Whenever a dinosaur leaves the battlefield, create
2 Fossil tokens (Fossil tokens= {x}).
Power and toughness = {x}, unblockable, vigilance
ETB: Destroy all dinosaurs and then deal {x}
player damage to all players.
Extinction {r}{g}{w}: Deal X damage to all creatures
and then sacrifice fossil tokens
equal to 1/2