Dreadmouth Leviathan

This spell costs {x} less to cast, where X is the
number of Leviathan creature cards in all
graveyards.
When Dreadmouth Leviathan entes the battlefield, it deals X damage divided as you
choose among any number of target creatures.
"Don't let its thin, mummified skin fool you. It's
nothing but a living, murderous nightmare!"
—Dashforth