Bumfo, Crypt Raider

When Bumfo, Crypt Raider enters the battlefield,
each opponent mills the top card of their library
until a player mills an artifact card this way.
{b/r}{b/r}{u}{x}, {t}: each opponent mills X. Then you
may put an artifact card with converted mana
value X or less from an opponent's graveyard
unto the battlefield tapped under
your control.