Mirrormade + Estrid's Invocation + Oblivion Ring + Sage of Mysteries

Mirrormade

Estrid's Invocation

Oblivion Ring

Sage of Mysteries

How it works

1. Cast Mirrormade by paying 2.
2. Mirrormade enters the battlefield as a copy of Oblivion Ring, triggering Sage of Mysteries and itself.
3. Resolve the Altar of Brood trigger, causing target player to mill two cards.
4. Resolve the Mirrormade's trigger, exiling Oblivion Ring.
5. Cast Estrid's Invocation by paying 2.
6. Estrid's Invocation enters the battlefield as a copy of the Oblivion Ring copy, triggering Sage of Mysteries and itself.
7. Resolve the Altar of Brood trigger, causing target player to mill two cards.
8. Resolve the Estrid's Invocation's trigger, exiling Mirrormade.
9. Mirrormade leaves the battlefield, triggering itself, returning Oblivion Ring from exile to the battlefield.
10. Sage of Mysteries and Oblivion Ring trigger.
11. Resolve the Altar of Brood trigger, causing target player to mill two cards.
12. Resolve the Oblivion Ring trigger, exiling Estrid's Invocation.
13. Estrid's Invocation leaves the battlefield, returning Mirrormade from exile to the battlefield.
14. Repeat from step 2 until each opponent has an empty library, then end the loop by choosing to copy or exile any other permanent.