Cthulhu, the Great Dreamer

Whenever Cthulhu, The Great Dreamer enters
the battlefield from anywhere other than exile,
exile it.
Whenever Cthulhu enters the battlefield or
attacks, exile the top five cards of target players library, then exile each player's graveyard.
{10}{u}{b}: Return Cthulhu from your graveyard to
the battlefield tapped. This ability costs {1} less to
activate for each card put into a graveyard this
turn.