Lethe Lake Leviathan

Lethe Lake Leviathan's power is equal to the number of instant and sorcery cards in all
graveyards.
Lethe Lake Leviathan's toughness is equal to the number of creatures in all graveyards.
Whenever a player casts a spell, that player mills the top X cards of their library, where X is equal
to that spells converted mana cost.