Myr Underworld

Myr Underworld enters the battlefield tapped
unless you exile a myr creature card from your
graveyard as it enters the battlefield.
{t}: add {c} to your mana pool.
{t}, exile a myr creature card from your graveyard:
Put a number of 1/1 colorless Myr artifact
creature tokens onto the battlefield equal to
the exiled creature's converted mana cost plus
one.