Karthus, The Deathsinger

Karthus, The Deathsinger cannot take combat damage if there are 10 or more creatures in each players graveyards.
Enemy creatures have -0/-X where X is equal to the total number of creatures in your graveyard.
{t}, Sacrifice Karthus, The Deathsinger: Deal 10 damage to all enemy creatures, and deal 5 damage to all enemy players.