River Kelpie + Mikaeus, the Unhallowed + Altar of Dementia

River Kelpie
Mikaeus, the Unhallowed
Altar of Dementia

Requires
All permanents on the battlefield.
Steps
  1. Activate Altar of Dementia by sacrificing River Kelpie
  2. River Kelpie's persist and undying abilities trigger
  3. Resolve the persist trigger, returning River Kelpie from your graveyard to the battlefield with a -1/-1 counter on it
  4. River Kelpie enters the battlefield, causing you to draw
  5. The undying trigger fails to resolve because River Kelpie is not in your graveyard
  6. Resolve the Altar of Dementia trigger, causing target player to mill cards equal to River Kelpie's power
  7. Activate Altar of Dementia by sacrificing River Kelpie
  8. River Kelpie's undying and persist abilities trigger
  9. Resolve the undying trigger, returning River Kelpie from your graveyard to the battlefield with a +1/+1 counter on it
  10. River Kelpie enters the battlefield, causing you to draw a card
  11. The persist trigger fails to resolve because River Kelpie is not in your graveyard
  12. Resolve the Altar of Dementia trigger, causing target player to mill cards equal to River Kelpie's power
  13. Repeat
Results
Infinite card draw. Near-infinite mill. Near-infinite ETB. Near-infinite LTB. Near-infinite death triggers. Near-infinite sacrifice triggers.