Cast Mirrormade by paying {2}{U}.
Mirrormade enters the battlefield as a copy of Oblivion Ring, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Mirrormade's trigger, exiling Oblivion Ring.
Cast Estrid's Invocation by paying {2}{U}.
Estrid's Invocation enters the battlefield as a copy of the Oblivion Ring copy, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Estrid's Invocation's trigger, exiling Mirrormade.
Mirrormade leaves the battlefield, triggering itself, returning Oblivion Ring from exile to the battlefield.
Sage of Mysteries and Oblivion Ring trigger.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Oblivion Ring trigger, exiling Estrid's Invocation.
Estrid's Invocation leaves the battlefield, returning Mirrormade from exile to the battlefield.
Repeat from step 2 until each opponent has an empty library, then end the loop by choosing to copy or exile any other permanent.

Infinite mill

Infinite self-mill

Mirrormade

Estrid's Invocation

Oblivion Ring

Sage of Mysteries

Cast Mirrormade by paying {2}{U}.
Mirrormade enters the battlefield as a copy of Oblivion Ring, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Mirrormade's trigger, exiling Oblivion Ring.
Cast Copy Enchantment by paying {2}{U}.
Copy Enchantment enters the battlefield as a copy of the Oblivion Ring copy, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Copy Enchantment's trigger, exiling Mirrormade.
Mirrormade leaves the battlefield, triggering itself, returning Oblivion Ring from exile to the battlefield.
Sage of Mysteries and Oblivion Ring trigger.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Oblivion Ring trigger, exiling Copy Enchantment.
Copy Enchantment leaves the battlefield, returning Mirrormade from exile to the battlefield.
Repeat from step 2 until each opponent has an empty library, then end the loop by choosing to copy or exile any other permanent.

Copy Enchantment

Cast Estrid's Invocation by paying {2}{U}.
Estrid's Invocation enters the battlefield, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Estrid's Invocation's trigger, becoming a copy of Oblivion Ring and exiling Oblivion Ring.
Cast Copy Enchantment by paying {2}{U}{U}.
Copy Enchantment enters the battlefield, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Copy Enchantment's trigger, becoming a copy of Estrid's Invocation and exiling Estrid's Invocation.
Estrid's Invocation leaves the battlefield, returning Oblivion Ring from exile to the battlefield.
Sage of Mysteries and Oblivion Ring triggers.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Oblivion Ring's trigger, exiling Copy Enchantment.
Copy Enchantment leaves the battlefield, returning Estrid's Invocation from exile to the battlefield.
Repeat from step 2.

Infinite ETB

Infinite LTB

Cast Mirrormade by paying {2}{U}.
Mirrormade enters the battlefield as a copy of Oblivion Ring, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Mirrormade's trigger, exiling Oblivion Ring.
Cast Clever Impersonator by paying {2}{U}{U}.
Clever Impersonator enters the battlefield as a copy of the Oblivion Ring copy, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Clever Impersonator's trigger, exiling Mirrormade.
Mirrormade leaves the battlefield, triggering itself, returning Oblivion Ring from exile to the battlefield.
Sage of Mysteries and Oblivion Ring trigger.
Resolve the Altar of Brood trigger, causing target player to mill two cards.
Resolve the Oblivion Ring trigger, exiling Clever Impersonator.
Clever Impersonator leaves the battlefield, returning Mirrormade from exile to the battlefield.
Repeat from step 2 until each opponent has an empty library, then end the loop by choosing to copy or exile any other permanent.

Clever Impersonator

Cast Estrid's Invocation by paying {2}{U}.
Estrid's Invocation enters the battlefield, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Estrid's Invocation's trigger, becoming a copy of Oblivion Ring and exiling Oblivion Ring.
Cast Clever Impersonator by paying {2}{U}{U}.
Clever Impersonator enters the battlefield, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Clever Impersonator's trigger, becoming a copy of Estrid's Invocation and exiling Estrid's Invocation.
Estrid's Invocation leaves the battlefield, returning Oblivion Ring from exile to the battlefield.
Sage of Mysteries and Oblivion Ring triggers.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Oblivion Ring's trigger, exiling Clever Impersonator.
Clever Impersonator leaves the battlefield, returning Estrid's Invocation from exile to the battlefield.
Repeat from step 2.

Cast Copy Enchantment by paying {2}{U}.
Copy Enchantment enters the battlefield, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Copy Enchantment's trigger, becoming a copy of Oblivion Ring and exiling Oblivion Ring.
Cast Clever Impersonator by paying {2}{U}{U}.
Clever Impersonator enters the battlefield, triggering Sage of Mysteries and itself.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Clever Impersonator's trigger, becoming a copy of Copy Enchantment and exiling Copy Enchantment.
Copy Enchantment leaves the battlefield, returning Oblivion Ring from exile to the battlefield.
Sage of Mysteries and Oblivion Ring triggers.
Resolve the Altar of Brood trigger, causing each opponent to mill a card.
Resolve the Oblivion Ring's trigger, exiling Clever Impersonator.
Clever Impersonator leaves the battlefield, returning Copy enchantment from exile to the battlefield.
Repeat from step 2.

Combos with "Sage of Mysteries"