Portal to the Underworld

When this artifact enters the battlefield, add {b}{b}{b}{b}{b}{b}.
{b}{b}{t}, Choose one: Exile target creature you
control. At the beginning of the next end step,
return that creature to the battlefield under your control, except it's a
demon in addition to it's other types, has flying
and has base power and toughness 6/6. Or, exile
target creature.