Demogorgon, Abyssal Demon

This card gets half of X as its power and
toughness, where X is the number of Killer
Plainswalkers in your graveyard.
{t}{4}{b}: Target Killer Plainswalker, send it to the
exile face down. When Demogorgon, Abyssal
Demon is sent to the graveyard in any way,
cast the face down Killer Plainswalker.