How can you create a combo with Dualcaster Mage, Extract from Darkness, Phyrexian Altar?
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[START OF COMBO]This combo can be formed with Dualcaster Mage, Extract from Darkness, Phyrexian Altar

Color identity: UBR
Mana cost: {4}{U}{B}{R}{R}
Steps:
  1. Cast Extract from Darkness by paying {3}{U}{B}.
  2. Holding priority, cast Dualcaster Mage by paying {1}{R}{R}.
  3. When Dualcaster Mage enters the battlefield, it triggers, copying Extract from Darkness.
  4. Activate Phyrexian Altar by sacrificing Dualcaster Mage, adding one mana of any color.
  5. Resolve the copy of Extract from Darkness, causing each player to mill two cards, and you to return Dualcaster Mage from your graveyard to the battlefield.
  6. Repeat from step 3.[END OF COMBO STEPS]

Result:
  - Infinite colored mana
  - Infinite death triggers
  - Infinite enters the battlefield
  - Infinite leaves the battlefield
  - Infinite magecraft triggers
  - Infinite mill
  - Infinite sacrifice triggers
  - Infinite self-mill
  - Put all creature and planeswalker cards from each opponent's library and graveyard onto the battlefield under your control[END OF COMBO RESULT][END OF COMBO]

Other prerequisites:
  - Dualcaster Mage must be in your hand
  - Extract from Darkness must be in your hand
  - Phyrexian Altar must be on the battlefield
[END OF COMBO PREREQUISITES]
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