Umbris, Fear Manifest, Opposition Agent, Hive Mind, Mana Severance, and Chainer, Dementia Master

Prerequisites

• Umbris, Fear Manifest on the battlefield
• Opposition Agent on the battlefield
• Hive Mind on the battlefield
• Mana Severance in hand
• Chainer, Dementia Master on the battlefield
• 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
• plus an additional for each opponent you have available

Steps

1. Cast Mana Severance by paying
2. Hive Mind triggers, causing each opponent to copy Mana Severance
3. Resolve each opponent's Mana Severance, searching for and exiling all land cards from their libraries
4. Resolve your Mana Severance, choosing not to exile any land cards from your library
5. Activate Chainer by paying and three life, returning a creature card from your graveyard to the battlefield as a black Nightmare in addition to its other creature types
6. That creature enters the battlefield, triggering Umbris
7. Resolve the Umbris trigger, revealing no lands from an opponent library, exiling their library
8. Repeat from step 5 for each opponent

Results

• Exile each opponent's library
• Near-infinitely large Umbris