Chainer, One with Nightmare

Each Nightmare creature card you own that
isn't on the battlefield has madness. The madness
cost is equal to its mana cost. (If you discard a card
with madness, discard it into exile. When you do, cast
it for its madness cost or put it into your graveyard.)
Whenever a Nightmare enters the battlefield
under your control, if you didn't cast it from your
hand, each opponent loses life equal to the
number of Nightmare creatures you control.