Your life total is greater than three times the number of opponents you have.
You have at least a number of creature cards in your graveyard equal to the number of opponents you have.
Mana Needed
({1} magic symbol) ({U} magic symbol) plus an additional ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) for each opponent you have available.
Steps
Cast
Mana Severance by paying ({1} magic symbol) ({U} magic symbol) .
Hive Mind triggers, causing each opponent to copy
Mana Severance.
Resolve each opponent's
Mana Severance, searching for and exiling all land cards from their libraries.
Resolve your
Mana Severance, choosing not to exile any land cards from your library.
Activate
Chainer by paying ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) and three life, returning a creature card from your graveyard to the battlefield as a black Nightmare in addition to its other creature types.
That creature enters the battlefield, triggering
Umbris.
Resolve the
Umbris trigger, revealing no lands from an opponent library, exiling their library.