Celestial Gatekeeper, Dual Nature, and Phyrexian Altar

Prerequisites

• Celestial Gatekeeper in hand
• Dual Nature on the battlefield
• Phyrexian Altar on the battlefield
• available

Steps

1. Cast Celestial Gatekeeper by paying
2. Celestial Gatekeeper enters the battlefield, triggering Dual Nature's first ability, creating a token copy of Celestial Gatekeeper
3. Activate Phyrexian Altar by sacrificing the nontoken Celestial Gatekeeper, adding
4. Celestial Gatekeeper dies, triggering itself and Dual Nature's second ability
5. In response, activate Phyrexian Altar by sacrificing the Celestial Gatekeeper token, adding
6. The Celestial Gateekeeper token dies, triggering itself, exiling itself and returning Celestial Gatekeeper from your graveyard to the battlefield
7. Celestial Gatekeeper enters the battlefield, triggering Dual Nature's first ability, creating a token copy of Celestial Gatekeeper
8. Resolve the nontoken Celestial Gateekeeper's trigger, failing to exile itself and returning up to two more Birds and/or Clerics from your graveyard to the battlefield
9. If you returned a creature this way, Dual Nature's first ability triggers, creating a token copy of that creature
10. Repeat from step 3
11. Resolve all triggers from Dual Nature's second ability, exiling all tokens with the same name as a creature card that left the battlefield during this combo

Results

• Infinite blinking of Bird and Cleric creatures
• Infinite colored mana
• Infinite death triggers
• Infinite ETB
• Infinite LTB
• Infinite sacrifice triggers